The Case of the Mysterious Disappearing Buttons

I have made some navigational buttons from pngs (shown below). They all have three triggers on them, a jump to slide, a hover state  (highlighted) and a clicked state (shadowed). If I navigate away from the page, and come back to it - Once the 'Heading 1' button is clicked again, it resets the narrative on the slide, but disappears.  

This seems to happen frequently... I have tried deleting them and starting from the beginning, and using no hover or clicked states, but still get the same problem - if anyone has any suggestions I'd be grateful for the help! 

Thank you in advance!

Em