What do you do when someone locks up against your technique because they are either in pain or simply resisting on purpose? This is a hypothetical question by the way.
I agree with Katherine regarding the issue of pain, and my short answer to them resisting my technique is to find a way of making it work anyway.
My longer answer is that, if they're committing to the attack and not so much better than you that they can just shut you down at will anyway, then I would check the attack sometimes by letting them get me (more so with suppressions than strikes of course). A lot of times it seemed like there was an anticipatory movement (a change of the attack, essentially) which revealed itself when I did this. Also, it usually got aite/uke to commit more purely to the suppression so I could actually deal with it instead of some other movement. Other times, where my ability allowed for it, I would simply adjust to their new attack and finish as much of the sequence as possible.
Different dojos train differently of course, but I remember one of the great teaching/learning tools I had was atemi. If I'm not controling their center and they can begin to make me work too hard for my results, then an elbow would tend to find its way into aite's ribs, or a fist would appear in front of aite's face, or something similar. When uke responded to that I would regain control. In the case of the elbow to the ribs it was a more physically direct way of creating kuzushi. In the case of the fist toward the face it was more of a way of diverting their attention, splitting their power that way...That's my sense of it at any rate.
If I'm still stuck and aite insisted they weren't doing anything wrong, then I would ask sensei to observe us trying (if he hadn't already come over on his own account) and he would correct who ever appeared to need correcting, which would be both parties sometimes.
Overall, my sense is that if uke can resist, I'm not doing the technique right. From the moment of physical contact, nage should physically be in charge. Of course, being a relative beginner, I'm not going to be doing waza quite right anyway, but I remember this understanding served me pretty well and kept me focusing on the real issue which includes learning how to work with what you've got to make what you need/want. Fortunately we mix up who we train with enough that even if I was partnered with someone who tended to resist my efforts and I didn't feel like I was able to learn much, I would eventually get to practice with someone who had high enough skill to show me what I needed to learn. Then the next time I ran into something similar I was more prepared and, appropriately enough, more effective.