It's really late and hot here and I'm finding it hard to concentrate so I apologise if I got this wrong but it sounds like it could be something to do with where the "tap" for the send is coming from (pre fader post fx etc..) OR...
could be something to with your current pan laws? remember that even sends can have there pan laws changed and this can still affect your send level on certain pan laws even if the track and sends are panned to the middle.
sorry if that's not the answer.. it's just to flipping hot!
