tl;dr Balance breaking, timing, and power/speed is what you are looking for.
The answer depends what type of training you: kata, randori, or shihai.
In kata, you are supposed to learn basic movement. As such your uke should help you and not hinder you. A quick word with them to ask "why is this not working?" should fix it. Although I suspect that you are missing something in the technique: balance breaking and speed of movement is all important there but when you start learning you have neither. Do it slow, making sure you get the movements right. Your uke should be giving you feedback, if not ask for it!
This is your opportunity to understand how the balance is broken for this technique. As you get more and more familiar with that, the speed of entry and timing come into play.
In randori, depending on the intensity, is where you get to make that kata your own: You have to get a good balance breaker. You have to get good speed. You have to get good timing. All the things you learned in the kata phase. And sometimes, none of those things will help! Some techniques are not suitable for you to use depending on your and uke's body types. Needless to say, your uke should be helping you there as well. After all, these are still learning/teaching exercises.
Of course, you get feed back from randori into kata! Randori does teach you when the timing worked, and how to apply that balance breaker. Thus,
<recursion> kata makes your randori better which makes your kata better which makes your randori better which ...
In shihai, you are on your own! Uke's job is to fully resist and potentially counter your technique. This is hard. You cannot get there unless you have a very good understanding of all the above steps. Do not try to run before you can crawl. ^_~
In the example you give of a shomenuchi attack, you need to get in as the shomenuchi is coming up. Once it is going down (the attack proper), you're too late. A very fast and speedy entry will give you a good balance breaker. When (if) uke is off balance, it does not matter how strong they are: they go where you place them! Of course, that requires some power on your part. Again, we are coming back to movement.
The sankyo pin... If you have small hands, chances are that this is not a pin for you. You should be able to do it in kata but do not bother is randori/shiai.
Finally, if you really want to get this to work: grow stronger...