It is very hard to find good a good Uke.
And it drives me nuts.
We have many bad Uke in our dojo (we do have some good), what we try and do is attack them the same way they are attacking us. sometimes that helps.
We have some that are a bad uke, then they complain that you cant do the on them.
So then you have to explain to them that it is the fact that their is no attack their. no need to do a technique if their is no attack.