As a teacher involved in teaching other Aikido teachers can I just say thank you for starting this thread.
I think there are two issues:
1) It may sound surprising but a lot of teachers aren't secure enough in their Aikido to experiment whilst teaching. This is the "If I can't do this (different) Ikkyo every body will think my Aikido is no good and so will stop coming to my class" syndrome.
2) I've made it and so I dont need to learn syndrome.
In my experience the former is the most prevalent and after a while it's difficult to get out of that mind set.
There is also sometimes a problem of perception. If the teacher is trying to perfect a technique he or she can sometimes lose the big picture. In other words the teacher may concentrate on one small aspect of the technique but the students wish to concentrate or need to develop a different aspect.
In essence this is a really difficult pitfall to avoid and if anyone has any suggestions how to avoid please let me know as I'm still trying to find an answer.