Synchronised clips Audio Missing

Hi - I'm having a problem with Synchronised clips. Every now and then they show up with a yellow exclamation mark and a big red bar where audio should be. It is frustrating as other times everything is ok. Not sure what I'm doing wrong or how to fix it. Any suggestions would be great -  thanks.

That red bar/exclamation mark can denote a couple of things... either the audio file itself isn't where FCP thinks it should be, or you've got an effect that isn't available. Can you post a screenshot?