Bookmark function not returning to correct place

Hi, I have made a series of eLearn modules in Storyline 2, which get uploaded onto an LMS. 

The problem is, when a user returns, they click "yes" to return to where they left off, then, they are almost always directed to the wrong place in the module.

Sometimes a user even gets to the final slide, but when they return, it is directing them to a slide at around the 60% mark.

I'm pretty sure when they are finished, it shows up as complete in the LMS, but to the user it SEEMS like their progress hasn't been recorded if the module shows up at 60% when they revisit. So completion is recorded correctly, but bookmarking isn't working correctly. This is occurring in all browsers.

We have other eLearn modules uploaded on the LMS, also made in storyline which don't encounter this problem - so we know it is something to do with how the files are set up. I don't have access to those working storyline files though, as they were made externally.

My output option is SCORM 1.2, and in the reporting settings, it is set to "report status to LMS as passed/incomplete". In the tracking settings it is on "Track using number of slides viewed: 54/54". The option to track using quiz result is greyed out (I don't have a slide set up to show them their results). (Not sure if this info is relevant to the problem or not.)

Please help!