some of my vocal audio tracks not playing back...help

[superdood]

Hi, I just loaded one of my sessions that was basically mixed to liking already. I click play and all the music is there, but most of the vocal tracks are not playing back.
I looked in edit screen and the waveforms are there and the automation appears to be working right in terms of volume and mutes. The green level meters on these tracks are not showing any signal at all. I'm fairly new to pt, but this is the first time I've run across this problem.
Again, when I last saved it, everything played back fine with all the tracks working.
Any ideas??

Thanks,
Steve