Convert to Mono wipes out audio?

Rushing to get through a final sound clean-up today...

Went back into my multi-track project file, lots of clips from FCP came across as dual mono with all the audio to the right, so I've been selecting them, opening in Editor and then choosing convert to mono. I know this is destructive to the original QT files, but I was in a hurry and didnt want to open every clip as a project. Anyway, on most clips this conversion goes fine but on a couple now it just wipes out all the sound! I can still see Waveforms and the volume is set 0db but there is no volume. The QT file in the finder is also muted out. Is this a bug? Is there any way to repair the clips without re-capturing??

10.4.6, FCS 5.04, G4 QS Dual 1ghz