I have an audio track, many tracks, actually, which will only pan in one direction. Panning one way works, panning the other way just moves it center. Here is what I've tried:
 

• Soloed track
• Converted track to mono (which I wanted to do anyway)
• Bypassed all audio effects
• Re-routed output directly to sound card output
• Removed Sends

Now I hear the original dry track, directed directly to the sound card output, in mono, with no effects. Still will only pan between right and center. If the track's Pan is centered, the track plays at 50% right. When I pan full left, the track sounds in center. What's weird is that other tracks have the same problem, but some of them only pan between LEFT and center!
 
If I output the track to the master bus, it pans correctly when I turn the master bus's pan.
 
I feel something is terribly wrong here and I'm only seeing a glimpse of the bigger problem through this panning issue.
 
Don't know if this is relevant, but yesterday I spent a long time adjusting levels and making some volume envelopes, only realizing at the end of the day that I'd been in Envelope Offset mode the whole time. This morning I sat down to re-assess all the levels but not in the Offset mode.
 
What might be causing this panning issue?

What plug-ins do you have on the track?  I've found that certain plug-ins seem to confuse SONAR and lock it into some weird mode with relation to mono/stereo that is similar to what you describe.  This is the case even if you bypass the plug-in.  

Sonitus EQ, also Sonitus Delay, which I'm not using and is disabled and was just left in there.

Hi. You should not happen to have a stereo file (or fx) in a mono track, or vice versa? I have seen cases where that has been an issue. Sorry if this is obvious.
All the best.

Are you inadveertaintly routing it to the wrong mono output on your soundcard?
 
Is there a hidden track or clip pan automation envelope that you are not seeing?

Sounds like you turned on offset mode and then changed the pan.  Then if you turn offset mode off, that pan effect will remain, even though you brought the pan back to center.  It's a very confusing mode in Sonar since just toggling it off doesn't undo the changes you made while it was on (and the change becomes sort of invisible).  Since the command is mapped to 'O' on the keyboard it's not uncommon to turn it on by accident.  If you bring up the Sonar help and type in "offset mode" you can find some topics on it.  Also search here on the forum. 
 
I'm not really 100% sure how to completely remove offset mode changes once you have made them.  If you toggle offset mode on (button on the Mix panel in the control bar) you can look at all your pan's and volume sliders and see if they are changed and change them back, offset mode puts some +/- signs next to things it can effect.  This is what I've done but their might be an easier way.
 
 

Yes, your method is perfectly workable IF the problem goes back to using offset mode.
 
OP: Double check that the track is actually set to mono interleave in the Track Inspector.
 
I just converted a Stereo Clip to Mono and yep, the interleave stayed as Stereo

msorrels got it right, the problem was setting a bunch of pans while inadvertently in offset mode. Simple matter of returning to offset mode, centering the pans, and turning offset mode off again. 
 
I'm new to offset mode, and while I can see the utility of it in theory, it is yet another good-idea-but-confusingly-executed Sonar feature. I'll stay away as much as I can. Thanks everyone!

Offset mode is very useful but I think Cakewalk failed at "finishing" it.  There needs to be some project level control features and some sort of visual indication it has been applied/is effecting a project. 
 
I'd think some sort of text/color-status in the control bar warning Offset mode changes are being applied and a menu item to reset/clear all offset mode changes would go a long way to making it work.

Offset Mode on: