This specific situation is excellent illustration of quite common mechanism that happens in many aikido dojo.
The problem is that without seeing the situation, we can not say which one of two possible mechanism was at work:
* Bad Uke - knowing the technique in advance and nullifying it in a non realistic manner, and/or during a specific type of training that should be more cooperative.
* Bad Tori and worse response from the Sensei later - Expecting success regardless of the quality of execution, and always putting the blame of failure on Uke Ego.
Both possibilities exist here, and reality could even include some combination. After reading the first post, I was quite certain the situation corresponded more to the second, but the later post changed my mind towards the first.
Personally, the environment I train in typically tends towards applying too much Resistance (a matter of the common Israeli mindset, when thinking of S.D. compared to the Japanese, every beginner has to show he can resist the technique in the first few lessons, and we tend to keep something of this later on).
It took me several years to realize that cooperative training does have an advantage at developing important aspects of Aikido, provided it is done in the right amount (there is such a thing as too much, in both cases). My solution is that even when I cooperate as Uke to a newer student, I would still point the mistakes and openings to him, and if he does not understand, I will show him. When I practice i try to select my Uke so I would be able to train the element I wish for in that practice - Resistance, or softness (selecting one of the ladies who are significantly lighter then me and trying to feel every slight movement and respond correctly to it).