Just try the following:
Create a new layer above all others. Then drag the slideshow onto it. What is the result?
You can control transparency in the upper command strip using the "opacity" slider. Klick through the slideshow parts and have an eye onto the value displayed there.
If this has no effect, it would be helpful, if you upload the Muse-file to a cloud provider (for example dropbox) and post the link here.