I recently got jumped in a parking lot by 3 guys, but when I tried to use every move I know (which is a lot; I.E. sidekick, elbow strike, knee strike, straight punch) and it did NOTHING!!! My Krav Maga instructor says I hit hard enough, but my strikes, grapples, and throws did NOTHING!!! How do I deal with this?
The three most likely reasons this happened:
Your assailants were too strong
A common myth of martial arts is that they are an equalizer. If you are fighting someone who is much larger or stronger, you can have the best technique in the world and still fail. There was a video I saw a while back with a sub-100 pound master against a guy who was over 300 pounds and muscular. The master could punch as hard as he tried, even tried using his entire body in a drop kick, and could not budge the other guy. There are techniques you can use to try to equalize the situation, from weapons to targeting vulnerable spots on the opponent, but in general, if people are bigger and stronger, you may not be able to beat that with technique.
As a side note, someone hyped on drugs or adrenalin may not feel pain, which is its own problem. A lot of techniques rely on pain versus actual physical incapacitation, but even broken bones don't necessarily stop an opponent if they're not feeling the pain.
Your technique may be bad
I don't know your instructor, or how familiar they are with combat, but they may be giving you too optimistic a report on the combat efficacy of your moves, whether because they want to impress upon you how effective their training is (thereby encouraging you to keep taking classes) or simply because they don't know what it takes to actually fight (unfortunately, an endemic issue in a lot of martial arts where they only test the techniques in controlled situations). Unfortunately, it really can be hard to gauge this in a safe way, but if you and a buddy (preferably one who doesn't train the same style) pad up and fight, do you find you can land the blows sufficiently? While the pads will reduce the force of the blows to reduce injury, the other person should still feel it. You can pick a threshold for what's a "if you weren't padded, this would probably take you out", but a general rule is that if you hit the padded person, and they're not pushed back a bit, they don't hurt a bit from the hit, you're not hitting them hard enough to actually hurt them in a fight. One of the other things this can teach you is how much your style depends on the other person not getting a chance to hit back, the "one-step" mentality where you unload one simulated technique after another with the assumption that, when you punch the guy, he's not going to immediately punch back, that when you grab his wrist, his other hand won't be pulling a knife, etc.
You may not have been operating at full efficiency
I don't know how often you train in a simulated combat situation, but adrenalin is a heck of a drug, and if you don't train to actually hit (padded) people, you're going to pull your punches subconsciously.
So, what can you do?
Well, first off, you're not going to win every fight. If you have an opportunity to avoid a fight by surrendering your wallet, consider if the $50 you have in there is worth the risk of brain trauma from getting curbstomped, or the medical cost of getting stitches are winning against an opponent with a knife (because they will cut you). Work on your ability to run, both in a straight line and getting over obstacles quickly. If you do train self-defense drills, make at least some of them involve making a decisive strike and then immediately turning heel and hoofing it. Building up your reflexes to get away from an attacker, rather than having a mindset that you will stay and beat them, will serve you well.
- There are two ways to stop someone: pain compliance and damage.
1a. Techniques that cause pain in a dojo often fail against resisting opponents once they get a little adrenaline or if they're just used to fighting and know how to shrug off a little pain.
1b. You didn't do enough to damage to stop your opponent. It's that simple. You don't shrug off a broken leg, for example.
There's no such thing as a one-hit stop. It's irrational to expect one strike or even a dozen strikes to stop someone in a real fight. People shrug off multiple stab wounds and multiple gun shots...and those are far more damaging than elbows. The best you can hope for is to make an opening to run away.
It's been my experience that Krav instructors don't really train people to fight well. They teach people to be aggressive, which is good, but I've not met many krav practitioners that hit as hard as boxer or land throws like a Judoka.
The solution to this is to engage in full contact sparring regularly at varying levels of power. Especially against people who practice a different art from you and who won't stop or back off when they eat a punch.
Strength and size beat good technique in unarmed fights more than martial artists like to admit. There are very good reasons why weight classes exist in combat sports.
Consider carrying a legal weapon and developing situational awareness. There's no reason you should have been jumped by 3 guys if you were paying attention to your surroundings.
Don't go stupid places with stupid people at stupid times.
Do you ever watch MMA or boxing? If you do, you may notice that many fights take more than ten minutes, involve dozens or even more than a hundred landed strikes, and the fighters are often not noticeably impaired until well into the fight. Of course, these athletes are exceptionally well-conditioned to take punishment and keep going; on the other hand, their opponents are equally well-trained to hit hard. The bottom line is, don't assume that a small number of landed strikes is going to affect your opponent. They might - a single well-placed, well-executed strike can cause a KO - but there's no guarantee.
So what do you do? Assuming there's only one assailant, and you cannot avoid the confrontation: work on techniques that put you in a position where it's harder for them to hurt you, and you can keep hitting them - e.g., put them in a Thai clinch and keep kneeing them; or take them down, get mount, and start dropping elbows. Sooner or later, there will be an effect. Or work on techniques that definitely end the fight, like chokes.
If there are multiple assailants, either use your strikes and kicks to create some space and run (by far the most promising approach - and as Macaco Branco points out, escaping and running should be an explicit part of your training). Or train hard to become a knockout artist, i.e., reliably deliver very hard strikes to the most susceptible points (chin, temple, liver). ("Training hard" probably includes competition in full-contact striking styles.) However, with multiple attackers, the most likely outcome is that you will lose if try to slug it out.
There are something like 31 rules for MMA fighters. 27 or so of them are rules to keep them from seriously injuring other fighters. Think about that in relation to street fighting. You are actually being trained to not seriously injure your opponent. That makes sense in training and sport. It does not make sense on the street and you will fight how you trained.
Even then, most decent folk have an inborn instinct not to actually hurt people. It is great for civilizing society, not for self defense. Martial artists may have very good general skills, but they may not have the right mindset and they may not have practiced versions of their techniques that can kill or maim.
Check out these references for in-depth discussions of this:
"When Violence is the Answer" by Tim Larkin
"Facing Violence" by Rory Miller
"Violence of Mind" by Varg Freeborn
These books are eye-opening.