Why can't I view release conditions for a quiz or assignment?

I often set up release conditions to ensure students complete short intro assessments in a unit before they can access resources and assessments. I've been having problems with some quizzes and assignments lately: students report they cannot view an assessment, even though they have completed the quiz that is a release condition (they met the condition). I try as a zstudent, and see the same thing. I edit the quiz, and "Add Release Condition" is dimmed and can't be clicked…I can't even see what release conditions I set, and can't adjust them later in the course. It's not all quizzes or assignments, but I've experienced it for both now. I've tried adjusting the grade book (turn it to undgraded and back), adjusted due and end dates (both assessments do have student submissions and have passed their due date but not close date - and when I adjust the due date, no change.)

Any ideas?