OK,so working away in this project. Simple project with two mono acoustic guitars, both of which I had panned (they are panned in an earlier version of the project so I’m not going mad). the two instrument tracks, a VST violin and piano.
everything was panned earlier and I just blasted out another rough mix with slightly different volume assuming setting would be the same - but I can’t pan ANYTHING in the track now. I’ve turned off all sends, inserts and none of the tracks, (audio and instrument tracks) will pan…this is very annoying. When i pan a guitar track now, I get a slight amplitude fluctuation when it centres and moves off again, perhaps this may help a diagnosis from the experts. I’m relatively new to all this so I feel a newbie error coming on.
But I haven’t switched settings or anything…at least not intentionally 
I’m using Cubase 6, MOTU Audio express and Windows 7. Stuff is still panning properly in the my projects.
…any suggestions? …is there some global MONO setting I may have hit by accident or something???