I have problems now and then with stereo and mono output, usually self induced from a plugin. Now I'm confused about this situation. I have disabled all fx and prochannel to prevent issues.
I have a synth that whose output in stereo and can't get a mono output from it (able to pan L and R).
I bounce it to a mono clip and solo. Still won't work from the mono clip to the guitar bus.
 
interleave=mono selected
track output is to stereo guitar bus to stereo 2 bus to stereo master (then to speakers)... won't pan
track output is to stereo 2 bus to stereo master... pans work
track output is to stereo master... pans work
Is there something i don't understand?

i made another track using the same VSTi and I get the same results... no panning to that buss.
I made a new bus and no panning to that bus.
Close Sonar and restarted with same results.
Updated Sonar to the 2016.08 version just to see if that helped, but no.

Did you remove all the effects from the guitar bus or just turn them off?

Its possible that the problem may exist in your audio interface output software.

I did move them as I tried many different options yesterday... but since you mentioned it I tried again after moving them to a different buss with the same result... and I have the latest saffire pro 24 drivers, but maybe I will reinstall them.  I don't understand tho how a track going to a bus in the same program, which outputs to the same channel in Saffire Mix panel works for some busses and not others. 
edit:  reinstalled saffire drivers and restarted computer... no diff

If you are using the track pan make sure to also set the send pan or set the "Follow Track Pan" on the send pan context menu.

Dumb question: Just want to verify that you're not using a pre-fader "send" to send the mono track to the guitar bus?  Or do you have the "output" of the guitar track going to the guitar bus?  Checking the following link, a mono file can be mixed in a stereo field as long as the TRACK is assigned to a stereo output.
 
https://www.cakewalk.com/Support/Knowledge-Base/2007013269/Selecting-Stereo-versus-Mono-Inputs-for-Audio-Tracks

all sends are removed... see pic for screenshot of the track.  remember, if I set the output to the 2-buss or master, it pans properly.  (fyi, i moved the guitar fx to the bass bus temp)
 
after doing this, i don't know how to upload a hi-res version of the screenshot so you can really see the details.
 

Can't see the pics. . . too low res.  Maybe post to another site and link or something?

The only thing I can think of that would cause that would be if the "Guitar Bus" has the "mono" button enabled. . .

here's a dropbox link... that should be full res if anyone wants to look at the screens:
 
https://dl.dropboxusercon...o-Pan%20Screenshot.jpg