There are many ways out of such a situation, I am sure you will learn them as time passes and your training develops.
but that is not really the issue. I have found a similar situation in a dojo I train in. There is one student who believes he is helping nage by resisting with all of his might, when really he isnt.. as I have demonstrated to him time and time again.
These people who resist technique instead of co operating, may be good to learn from at higher levels, but at earlier levels all they do is serve to reduce confidence within the art for the person who is having a hard time.
The person I know who acts in this way during training believes that he is creating a realistic situation in which to train. But, he has just watched sense demonstrate technique and so knows what is comming and can attack and resist in such a way to restrict that particular technique. I have found that if I change technique then that person ends up on the floor or pinned every time as the resistance he is using is useless for the alternitave technique I have chosen... which is obviously why I chose it.
but alternative technique is not really a solution. as through the ukes un co operative nature you are forced into bad ettiquette, by not practicing the technique which was demonstrated by the instructor.
I have found one way of dealing with it is to not actually do anything, just stand while uke grabs really tight and wait. he usually gets bored and relaxes, giving you the opening to do the technique that was demonstrated. after repeating this many times uke will either start co operating... or start doing all kinds of awful stuff :P
discuss training methods with your sensei, discuss your problems within the training without necissarily discussing specific people. hopefully your sensei will talk a little to his students on the role of uke and the reasons for co operation, especially during kyu grade practice