See comments on individual questions after answering correctly

Interesting idea. 

It may be complicated to implement, since I think many of the exercises and tests and quizzes generate different problems each time.
Maybe a first step would be a Questions section under the test/quiz/exercise.

A next step could be a button in the test/quiz/exercise that copied the current problem into a new Question.  The user would then be able to ask about the specific difficulty they had with the problem.  (Though that might encourage too much spam, where some users just press "Ask question" without adding their specific question.) (It might also be complicated to implement, since many of the problems have formatting in them that would need to be stripped out, and others have graphical components.)