Now I have updated to the mentioned version.
Firstly I'm a new bee to Captivate, I learnt captivate from this community and youtube.
I didn't set up any actions to track visited slides, I don't have a quiz in my course.
You mention that you have created the forward and back buttons at slide level. If I understand you correctly, does this mean you have added two buttons to each and every slide in your project? If you have a large project, you may have hundreds of buttons, and sometimes this can cause issues because you then exceed the number of interactive objects that can work without issue in a single project. (Don't ask me how many that is but I know it falls somewhere over 100.)
You can easily test whether this might in fact be the cause of your issue by just Hiding half the slides in your project to reduce the number of buttons. Republish and test your project with say just 40 or 50 slides.
If you currently have buttons on all slides, but you have your TOC on the Master Slide, why not just add the two buttons to the Master Slide. If Master Slide objects are set to be on top, then those two buttons should appear on all slides. If the actions on the buttons are only to go to the previous slide or go to next slide, then these are generic actions that can be used for buttons on a Master Slide.