Storyview in SL messing up

Hi, I keep having issues with Storyview in Storyline. I can work on the project for a while and then suddenly it will look like this (see image). This has happened on several of my projects now.

I try zoom in or out and still no slides appear. I doesn't let me click on a slide either, so it essentially just blocks me working. The only solution I find is saving it and then re-opening. Does anybody know whats happening?

I'm on Mac and using parallels to access. 

Cheers