Another thought - are you using a custom theme? It happened to me one day, too. As it turned out, in my custom theme, the feedback text was very light yellow on white background, so I just didn't see it on the screen.
Quiz
Quiz Feedback Working?
Гэтае абмеркаваньне было заблакавана, таму вы болей ня можаце адказваць на яго.