One possible solution is that the voice on the track is set to the same voice as another track above it, or was set to voice off the last time it was saved in TDM.You can tell this is the case if the color of the audio on the track is the same color as the audio on a track above it in the session.To fix the problem you will need to take the session back to a system running Pro Tools TDM, or back to 4.x to reallocate the voices so that all are on, and none are sharing.