It might be but it isn't likely. Did you close/reopen, close/logout/login/reopen on various browsers? Have you looked at the attempt tracking? Does your site have a timeout setting? If you open a scorm and then leave it sit, if the network times the user out and then they start doing it again, it won't remember anything done after the timeout, which is a huge pain in the butt, however, if you warn them to close if they might be away for a length of time. also, does your Captivate module report interaction data? That can help...
Is reporting set to default, report at end or custom. Reporting at the end might interfere...I've never personally used that setting. Also, check the LMS advanced settings...it sounds like the never send resume data is not checked (it resumes for you) but it never hurts to check.
Do you have a table of contents? If you do and it allows them to go to pages already viewed but not unviewed, you might be able to see if it remembers where they were but just doesn't open or if it isn't tracking at all. Have you logged in as them? Seen what they are doing?