Non-Functioning Submit Buttons

I seem to be having an inconsistent issue with my Submit buttons not functioning and freezing a course. The course will work as expected for 9 out of 10 individuals, then someone will have a non-functioning submit button. I have found a consistency - not among the course, browser, etc. It was built in Captivate and these courses functioned without issue previously. Nothing has changed. I've attempted republish, rebuild question at issue, neither has worked. Sometimes a clearing of history and cache works and other times it does not. Sometimes a register and reregister works and others not. I'm at a lost.

Any suggestions? Checked HTML tracker and nothing there.