First of all, you shouldn't be opening sessions on the same hard drive that your protools application is installed on. You can get a way with it for a minute, but over time it will really screw things up. You should be opening/creating sessions on a second high speed drive(i.e. external fire wire with 7200rpm bus speed). As for your question, cosign the above answer. Otherwise, sometimes protools gets inexplicably picky about recognizing files, especially a file created on another system, hard drive, or protools version. This doesn't happen too often but it could be the case here. Unfortunately, if this is true, you might have to create a new session and import the files and rebuild the thing. Tedious, yes.