Bookmarking problems

Has anyone had experience of their bookmarking playing up?

I have inherited a course from my predecessor which is a 117 slide storyline course, with multiple numbered sections. 

The bookmarking is playing up in certain areas of the course. It works fine on most screens, bringing you back to the correct screen, but around certain areas it seems to consistently bring you back to a couple of pages before, although it is not consistent enough to pinpoint easily.

It's almost as if once you've got past a certain page it's confusing it.

 

Can anyone help or give me an idea of what to look out for which might be causing this?

The only unusual thing I can see is that the numbered sections aren't visited in order, due to some branching presumably added in later in the build. But I assume this shouldn't cause this problem. 

Tracking completion occurs at 97 out of 117 slides, to take into account some optional Glossary slides and the branching mentioned above. Could this be affecting anything? Although I'm not sure what the alternative to this would be as I still need to use this method to track completion.

 

Any help or direction greatly appreciated 

Thanks

Charlotte