Josie, I've experienced this problem more than once. BAck about a year or more ago when I recorded my last CD project, I had some files that had this issue. TRAX is right... MP3's can sometimes cause this... it's not really their fault, after all, they ARE condensed files.

What I found that works best... is: WAV files. I used one of the big file sharing services to swap WAV's back and forth. Mind you, now, I did get some MP3 files that DID work... but when they didn't I had to switch to the LARGE WAV files to get them to sync properly.

I also had problems sometimes if other collaborators only recorded at a start and stop point in the song. It's best that if they are going to play on the 2nd verse, for example... have them hit record at the beginning of your song, and just sit back and listen and then when it's time for them to come in... have them come in and play, sing or whatever. That always worked WELL for me. WAV is the best. I've even heard good reports on WMA files, but I generally always stuck with WAV files.

Best wishes for best recording!