Frustrated... at one point of time, I do get frustrated. Things such as not being able to get the technique right, or why this and that don't work.
But those things don't bother me anymore, because it is something called "the learning process". At first, everything was only based on form, people just follow through. But as we progress up the ladder of knowledge, people that once we train with refuse to follow through anymore. This is caused by the need of truth and reality of the subject matter. Everything that seems to be the simplest became the hardest, such as Long-san's ikkyo training session.
Having an uke that gives me a hard time, teaches me a lot more, than having an uke that follows through. This is of course within boundaries of experience. I would never allow such attitude on a complete beginner, they need to be introduced to proper forms first. After their first grading maybe I would expose them little by little, showing why the forms were suppose to be done the way it was done. But when you're more than half way through the grades, it's time to face truth and reality completely.
The trick is not to get frustrated, remembering that you are still learning, no matter how high your grade is. Always have a mind of a beginner, don't get the head swelled up because of the belt color. Different partner, different things to learn, new things.