Playback not working on some patches

I recently bought Mainstage 3 to use on my MacBook Pro, and set up a new concert with 6 patches. I added backing tracks using the playback function - some mp3s, some wavs.

For some reason, on two of the tracks the playback is working perfectly fine but on the other four nothing happens when I click the 'play' button. The play button lights up but there's no sound and no signal on the channel strip. Looking at the settings, I can't see anything different on the patches that do work to the ones that don't.

Weirdly, when I actually open the playback app I can play the wavs/mp3s fine, but when I close the playback app and try playing again it doesn't work.

I'm starting to get worried as I have a gig to rehearse for and time is running out!

If anyone else has experienced this or has any suggestions I'd really appreciate it!