Keep in mind that uke is on his own journey and might be completely unaware that he/she is being difficult. Or maybe he/she is aware and is working on it but is just not there yet.
If I am consistently having a problem with a specific person I just ask my sensei how I should respond when in that situation. It is usually me that needs to change rather than my partner. Occasionally it is the other way around and sensei speaks to them about it or perhaps it is someplace in the middle. Any rate perfect ukes are few and far between. Which is probably as it should be.
That reflects my experience over and over. Learning to get out of my own way.
It's funny that I think of it more like the Uke from Heaven because training with either:
A: New person with little to no martial back ground let alone Aikido does not move/attack the way someone with even a few months of Aikido 'conditioning' will. So your waza has to be good to create the shape in Uke that you are looking for.
B: Sempai who is hopefully only trying to improve your waza by only blocking/resisting the holes in it but not to the point of countering it. This provides me the opportunity to improve my Kihon.
There will always be those situations where Uke does not even realize that they have grabbed or attacked just slightly incorrectly and so I usually just ask them to make the adjustment but carefully phrase it to not come across as "Like most new people, you attacked me wrong
but instead try for "I believe the intention behind the grab or attack for this particular technique 'Sensei' demonstrated was like this.' and if they are unable to (hopefully not unwilling to make the adjustment) then I am always happy to have Sensei come over and 'explain it to them'.
A little context about how and why the grab/attack way goes a long way towards good productive training. Even if it is only to allow Nage to develop their movement/balance through techniques more than 'this is what you do if someone manages to grab both your hands from behind you and hold them firmly at your side... because this happen to a friend of someone I know just last week...'