I'm having a problem with one of my Sonar mix sessions and haven't been able to find any answers or solutions to it so far.
 
I recorded a song in Sonar X3, session wrapped, mix prep done. I did a first mix, wasn't happy with it so I decided to go back and start again. So far, so good. I reloaded an earlier saved session from the start of the first mix and have rebalanced, levelled, eq'd etc. The problem I'm having is that the master bus is playing only mono signals despite being set to stereo. I've checked all the buses and going to the master - guitars/bass/drums/reverb sends etc - and they're all stereo buses. Of those, only the drum bus is sending a stereo output (I can tell because the L-R meters move differently, whereas all the other buses move with the L and R meters in perfect sync).
 
None of the panning controls on the tracks seem to be having any effect and I'm really not sure what the problem is. I've checked my soundcard and it's not that. I tried opening other sessions in Sonar, including the earlier mix of the song and they all work fine. During the course of this mix I did flip the stereo/mono interleave switch on the master bus back and forth quite a bit so I could check the mix balance in mono, but I'm not sure how that would have caused a problem itself, especially after switching it back to stereo.
 
Anyone had this problem before?
 
I'm using Sonar X3 in 64 bit, Windows 8, RME Fireface UC.

did you add any plugins that were mono only? That'd be my first guess without taking a look at the actual project

Hi, well I just tried that to check. I had a number of buss plugins running and turned them all off to check. Some were indeed mono, but that still didn't sort out the problem because even on the busses where I removed the mono plugins, and could see the L-R meters moving independently, the master bus was still functioning in mono. This is despite being set to stereo and having no plugins running.

Try cycling the MONO button on and off a few times. If it still persists, close an reopen Sonar. Sometimes functions fail to operate properly.
 
At the worst you may have a corrupted project that can't be fixed.

Will do. I've tried it a bunch of times but will have another go.

Try deleting the mono plugins as well. 

I've experienced a similar situation. Don't remember the details but could not find the culprit. Luckily, I had already exported the master. It was a very big project with tons of plugins and (ASAF) the problem started after I opened the old project for checking something without adding any new plugins.
I thought I'd gone deaf or crazy or something.

sharke
Try deleting the mono plugins as well. 

"That's a bingo!!"
 
It was one plugin in particular - Blue Cat's frequency analyst. I've been using 64 bit so can't use any waves plugins and have been using either bundled plugins from cakewalk, or else free stuff. It's the best freeware frequency analyst I've tried, but now I know the mono version has this weird side-effect on buses. Strange. None of the other mono plugins I used on stereo buses did that.
 
Thanks for the suggestion. Forehead-smacking time.

Just to clarify - I had to delete the plugin from any buses it was on, not just disable it.

For a frequency analyzer, try Voxengo's Span.