Back to the subject ...
As we return to the subject of resistant uke's and when is enough, enough ... yeah, I have hurt uke's who are not merely resisting for the sake of training but for prooving your technique does not work. I try to be more gentle in releasing my technique so nothing more than a sore muscle is felt these days.
Let me bring up the art of taking balance, creating distraction.
One of our traveling friends, Butch Chernofski, a traveling companion of Y. Yamada Sensei of NYC, consented to teach class while visiting LBI on holiday. He is not a big man, about 5 feet 8 inchs about 150 pounds but at times when proving a point his weight feels more like my 250 plus pounds.
Now, it isn't that the fact that he is heavy, but redirecting my energy to make it less effective. It is an old and sometimes difficult thing to learn, but part of taking the balance/ distractions learned with practice.
Amazingly enough, relaxing the body and arms while sending the power out your fingers beyond the balance of your uke is the key. What is really cool, but most difficult to get a grasp upon, is that it absolutely feels like nothing at all.
Once balance is taken, pain is inflicted, the body follows. If your uke is twisting and wriggling out of your practice technique, I guess it is time to inflict as little pain as possible to reawaken them to the fact that this practice of Aikido is indeed a Martial Art ... not just a squaredance.
Of course, if you don't have that expertise let someone with more experience awaken that person, but then it would also mean you need to work upon your own technique for practice also.
The biggest problem I have is usually people who have been in Aikido for a couple of years, who have made a decision that they are more skilled than their new partner, me. That is fine if they are, better practice for me, but usually it leads to them being hurt because I am being gentle as not to torque them out because they are wriggling, or trying to change the technique. OH, WELL!
Anyway. I want to advise you to use extension and push through the technique without using strength to the point you have taken the strength of your uke or redirected it.
It may seem like a rehash of an old simple foundation, but then that is also the magic of finding new things in Aikido.
As for resistent uke's ... try your best not to cause injury. Bring you instructor over to correct whatever is giving you trouble, I am sure the instructor will cause enough pain to reallign the offending student, or at least correct your practice to make resistant uke's more compliant.