Since the "Check Answers" button only appears for students when they can no longer submit answers, have you checked both an open set where the student has attempts remaining, and a set with no remaining attempts, or one where the due date has passed?
Re: Webwork warning: uninitialized value $checkAnswers; only in student viewby Danny Glin - Number of replies: 0
It looks like there is something wrong with determining whether to show the "Check Answers" button. One thought is that this might be related to permissions. Did you change any permissions either at the system level or the course level?