MODULE IS MARKED AS INCOMPLETE - CAPTIVATE 2019

My guess would be that even though your users THINK they may have visited every slide there is still at least one slide that they have not visited and this is the reaso why the module is not completing.

To be quite honest, I think it is almost always a bad idea to use slide completion as a success criteria.  There are just too many ways that this can fail and cause issues.  Additionally, using slide completion doesn't really guarantee that your learners actually learned anything or even that they were paying undivided attention to your content.  

If the original reason for choosing this method is that you wanted to make sure the learners saw enough of the content, then I prefer to just place a button at the end of the content that will pause playback and instruct the learner they need to click this button.  This button is set to have a score reported to the quiz and that is all the learner needs to achieve completion. 

You also need to make sure that the navigation is set up so that the learner must progress through the content and cannot jump straight to the slide containing that button. But by using this method I never have to resort to slide completions.

The better way to verify that is to include assessment components with appropriate difficulty so that only if the learner actually viewed the content would they then know how to pass the assessment.