Loaded up tracks to mix. Am unable to switch the interleave button on the Master bus from stereo to mono.
 
Anyone know why this might be?

What version of Sonar? Is this a new project? 

The project is an imported 44 track song downloaded from "The Mixing Secrets Library." I actually did a mix of the tune before using Music Creator 6, and everything was fine. I'm up sampling now on import, but can't imagine that is relevant.
 
I use X3 Studio now.
 
Also, when I originally posted, I wasn't aware of this, but it turns out that when I opened up another project this morning, I was able to switch up from stereo to mono on the Master Bus without issue. However, the original problem one hadn't changed in this regard. Also, and I didn't mention this before, I don't think, the individual tracks on the project show no problem switching up the interleave. It appears to be only on the Master Bus. I haven't created additional buses yet, so can't say if they would present a problem yet.

I'm not sure why one would want to make the master buss mono but that said it could well be due to the project itself. It could be due to an FX in the master buss. You may want to copy the tracks to a new project with a mono master buss. 
 
 

checking the mix in mono is a good practice

John
I'm not sure why one would want to make the master buss mono but that said it could well be due to the project itself. It could be due to an FX in the master buss. You may want to copy the tracks to a new project with a mono master buss. 

Checking the mix in mono is a really important thing for a number of reasons. The obvious one is to make sure the mix holds up on that little, single speaker radio sitting on someone's desk. Hell, I had a mono output television until about 2 years ago! lol The other reason is that it will quickly reveal serious phase problems which could be sneaking around in the middle of it all.

Do you have a mono plugin on the master bus? There is a bug related to the update of the interleave button, which could be related to your problem here.

sharke
Do you have a mono plugin on the master bus? There is a bug related to the update of the interleave button, which could be related to your problem here.

I do not. However, when I went back to the project a short while ago, other problems of unexplained origin flared up, like all the plug-ins graying out and not receiving signal. I am currently re-importing the audio files into a new project, so we'll see if it was just one of those cases of super-buggy-out-of-nowhere situations. Wouldn't be the first time the unexplained cropped up never to be fixed, but steered around instead!
 
I'll be posting back if the problem continues, of course.