Re: Difficult uke or bad technique?
My way of handling that sort of situation is to just work as best I can until sensei comes by and then look at sensei and say "I can't seem to make this work. Can you show me how to do it and help me figure out what I am doing wrong?" Most times sensei will use my partner to demonstrate. Partner if he really does have a problem with his ukemi will get corrected by sensei and I never look like I am accusing them of attacking me wrong. And if the problem really is me then sensei will see and correct that as well.
Between 4th and 3rd kyu is about when my teachers started saying well, if you can't do that technique because of the ukemi you are receiving then do something else. Now just past 2nd kyu I am beginning to see more of the thought on observing what it is ukes attack is requiring as a response and doing that rather than trying to force a specific technique to work when the attack might not warrant it. On the other side we also are spending a lot more time examining what uke's role is and how to improve our ukemi so that we can be a good training partner.
One observation I have made many times is that not enough attention is given to teaching people what uke's job is and how to be a good training partner so that your nage can learn and so that you can as well, while taking ukemi. Fortunately my teachers are giving more attention to that aspect of training. Might be something you could request as a subject for class some time.