The Next button does not work in preview

Hi,

I have had a problem with a couple of my courses. I will preview my course and out of no where, the next button will stop working. It was previously working fine, and I have it set up so the next button should jump to the next slide so there should be no reason for this to happen. The strange thing is, the slide which is impacted changes every time. For example, I load it to preview and slide A's next button will not work. I cancel the preview, then restart the preview and slide A will be fine but slide B's next button will then not work.

This has happened with two of my courses. My others seem fine.

It is the same when I publish the project.

Please help, my projects are due before Aug 3rd!

Thank you