Probably not the case but are there any special characters in the quiz answers that your LMS may not like?
For example, we use Saba as our LMS and had a problem with the % sign.
You may also want to review this:
» 9 Ways to Troubleshoot Articulate & LMS Issues - Articulate - Word of Mouth Blog
As the article discusses, we always find it helpful to use a browser sniffer (we use HTTP Analyzer) to help see where the problem.
If we see the data being sent back right to the LMS by the quiz, and not being acknowledged correctly as being processed, we start looking at the LMS or network.
If the quiz doesn't report back right to the LMS, then we start looking at the content.
Happened just this week. One of users had an issue with a course not completing. Fired up the AP course in the LMS and watched with the browser sniffer. Saw that the quiz results were not posting back correctly. The user was still using AP5/QM2 and we knew it could not handle hidden slides in the PowerPoint (known issue) and suggested he look for that. Sure enough, there was a hidden PPT slide and once he removed it, the quiz reported back just fine.
(Of course, Articulate addressed this for us with the new AP09, which can now handle hidden PPT slides.)