iMovie has 'lost' lots of my clips

Just trying to do a compile of old footage & I found heaps of my clips are marked as only being 0.01 in duration. Obviously when I try to play them nothing happens.

Uh-oh I thought, iMovie has somehow trashed the original footage so I looked in the iMovie events folder & the original footage is still there & can be viewed in QuickTime.

So I tried to import one of the clips & it did so no problem & it went back into the event it was supposed to be in.

What gives?

How can it not be seeing the files in its own source folder?

More importantly how can I fix this. Doing a faux import is just out of the question as it woukd takes hours & hours & will probably result in footage being duplicated.