Not using any automation.  Offset is not on.  Meters show pan hard left but sound from studio monitors is coming from both speakers.  If I mix down and play on another machine - pan seems to be working fine.  But this makes mixing difficult if the studio monitors don't reflect the pan.  What am I doing wrong?  HELP
THANKS all
 

How's the signal routing? Is it going out through a mono something, FX or Master Bus?
Is the "other machine" connected to the same monitors?
What kind of soundcard and monitors are you using?
We don't know what you're doing wrong because you don't tell anything about what you're doing :o/

Signal is going to the master out.  I mix it down and play the mixed down track on another machine not hooked up to the same monitors and I can hear the pan is working,  Using Presonus Firestudio soundcard with Yamaha monitor Running Sonar Professional 8.5 ona a windows 7 machine.  Pan used to work fine.  No idea what I may have done to make it all of a sudden not work.
Have to be away from computer for a while - when I get back I will try playing the mixed down version thru that soundcard and see if it pans properly
Appreciate the help
 

I'd start with your software mixer. It's the only piece that doesn't have any effect on exported audio.

Is it the same with headphones?

As said it has to be beyond the Master bus or the export would be mono. 
Dave is probably on it with the software mixer. My Focurite has one and a couple of times it's gone mono on me. I'm finding the Mix Control software a bit buggy.
Another part of the chain your not forthcoming with, is there an integrated stereo amp involved? A mono switch been flicked. Or are the Yamaha's powered. 

DUH!  Kicking myself in the backside now.  Obviously the software for the mixer.  Unfortunately I don't really understand the Presonus Firestudio manual - so I just spent an hour or so fooling around with various settings.  Finally found it - and now the track pans properly.  Hope I didn't screw up anything else in the mixing process with the changes I made.  Will find out soon.  I open this software maybe once since it has been installed.
 
Thanks to all for your responses - appreciate the support