tracks won't pan left

Hi Folks

I’m pretty new to cubase and have version 7LE.

My problem is that the audio parts in one of my tracks won’t pan left. As I try to pan left the volume drops until silent when I’m 100% panned left. They pan fine to the right. I recorded the tracks in mono and they are stereo out. On the mixing desk when the tracks are panned centre both the left and right bar of each track are illuminated as expected. When I pan left the left side bar gradually drops to zero.

Any thoughts?