Menu / slide greying out problem, driving me nuts!

Hi everyone, I've recently started making an e-learning module where the user can hopefully click on any menu item at any time. But when I preview or publish it, once a slide is finished, it is firstly greyed out on the menu...it only allows me to progress sequentially just once. When I then try clicking on a previous slide(s) to re-visit them, nothing loads and the whole slide content area becomes grey (no matter which menu slide I now click, none of the content then loads!)...and then I have to exit the whole project and re-start.

The player navigation is already set to "Free", and I've also tried the "Reset to initial state" option. I've just been making simple scenes (with normal slides, no video/sound)...I haven't messed with Layers or States at all. And I'm using the Trial version of Storyline at the moment (hopefully that's not the cause?)

If some kind soul or souls can help me, I would really really appreciate it!