So I send a pair of drum overheads, panned full L and R, to a stereo bus. At some point in the project, the bus stops seeing two feeds and will not respond to any panning of the tracks. It did initially, and when it turned mono, I did every conceivable variation of PC, VSTs, sends, including paring down to a bare bus, but still no response.

But then I'll insert a stereo bus, feed the overheads to it, and everything's back to normal. The really weird part is eventually the same thing will happen to that bus, even with different fx, sends, etc.

This is turning out to be more difficult than I expected.

Has anyone experienced or heard of this? Thanks in advance!

Did you inadvertently hit the stereo interleave button (looks kinda like the infinity symbol)?

Hi Treefight good to see you back. I would take the advice of Dave for this. I can't recall ever having a track or a buss switching interleave on its own. 

I experienced something alike recently. The reason was I missed the stereo-mono-switch of a compressor. I deleted and re-inserted the VST and forgot to check the settings.

Thanks you John, and thanks folks, and yes, I tried all of those things and more. Really a head scratcher. Unless I'm missing something obvious, such as is there such a thing as a mono bus and or a way to switch busses to and from mono? If so, I'm either forgetting something (not at PC now), or I've got egg on my face. Thanks again.

There's definitely no mono bus. There are stereo buses and surround buses and that's it.

Though as has been mentioned, you can force a bus into mono with the interleave button.
 

One other thing to try is disable all plugins using E.  If the bus goes back to Stereo you know you have a plugin that is forcing mono.

Did that too, went all the way too deleting all plugins, all interleave combos on the two panned tracks going in, nothing I haven't done a million times before without this happening! I wish one of the bakers could see this post, because it feels like somewhere signals are crossing deep in the guts of Sonar or... What? Having never seen this, and with it happening in all variations, including templates that otherwise work perfectly (but it'll happen with them too sometimes), there is absolutely no rhyme or reason.

It's so random, I don't even get (that) frustrated with Sonar, per se, any more when it happens, because it's almost like there's a poltergeist behind the curtain pulling strings. There are cumbersome, time-consuming, and temporary workarounds (insert new stereo bus, route the tracks to the new bus, insert the same exact effects from the old bus if I want to, and it works - but then sometimes THAT bus then does the same thing out of the blue!).

Ghost in the machine!

This sounds eerily similar to the problem I'm having.