Sometimes when I open a project it says it can't find the media and wants to know if I want to leave it offline. Restarting Vegas seems to fix the problem. Anyone else have this problem?

If I tell it the same file again by hand its fine, but that's usually a pretty big pain.

Any suggestions? All the files are in the same folder, and others work even when some can't be loaded. I haven't found a pattern yet. mp3, avchd & wav files in my project right now.