I noticed that background vocals on one song were sounding a little out of phase.  So I began to check each BG vocal track individually.  I have them panned 80% R-L.   But when I Solo'd them, they're all in the middle.   On one vocal track I tried panning L and R, going back and forth, but they didn't move from the middle.   I checked everything I know to check.
 
Suggestions?
 
Thanks!
Greg

Do you have any mono plugins on the BG vocals bus?

I have mono EQ in the Bus.   I guess that will do it.
 
Thanks!
Greg