Have you tried a reset/rescan of your plugins?
 
How about holding SHIFT while opening the project?
It will ask you to confirm each plugin instance.
Basically it tries to initialize(fix) your project file in case
some wires got crossed during the original event that
caused your problem.
 
It's great to have another compressor in a crunch but
Not knowing what happened and how to mitigate it
would drive me nuts. 
 
Mel

You seem to have the master, which is soloed, grouped with something. Turn off the Solo button in the Control Bar and the noise gate will work. 

First--thanks for the buss compressor info and link--this looks very useful
I've been resetting/rescanning a lot, to no avail.  Haven't tried the SHIFT opening maneuver.  Will do.
 
The solo switch on the bus was left over from a switching arrangement so I could A-B my mix status with a reference track.  I'll go into the big project and see if taking that stuff out helps.
 
Much obliged everyone; rebuilding the affected projects is not attractive at all.  I'll report back when I get a chance to try the solutions offered.

Removing the solo group switching fixed the gate problem (in more than one instance) in one of my other projects, so I will assume that's the snag--yay!  I don't need to know why; just glad to have it working.
 
still, I now have to go experiment with sidechaining the buss compressors because they look so interesting!
 

lloydjrose
Removing the solo group switching fixed the gate problem (in more than one instance) in one of my other projects, so I will assume that's the snag--yay!  I don't need to know why; just glad to have it working.

"Why" is a little more complicated, but you have to be careful about what you group, and if you hide any tracks using Track Manager, because you might end up muting something you don't want to mute by hitting solo. Glad it's sorted out, though!

One more comment...a lot of times, the first impulse people have is to re-install SONAR or whatever. This is rarely needed, and can make things worse because now you're back to defaults you may have changed.

Many thanks guru C--I've been a fan since PAIA days.
 
The reflex to re-install runs deep--thanks for clarifying that.  
 
Any suggestions (by anyone) on how to set up an A/B switching environment to easily reference commercial recordings while working on my amateur multitrack projects--would be greatly appreciated. I guess setting up the solo groups within the track environment is safer than using the busses...(?)

Exclusive Solo will do what you want. This documentation is for X3 but the functionality is the same. What's cool is you can easily A/B/C/D/etc. etc., not just A/B. I use this all the time to compare tracks, it's a great feature.

Craig, in most cases you can delete the X3 and the current help will be found so when
http://www.cakewalk.com/Documentation?product=SONAR%20X3&language=3&help=Playback.16.html
points to the X3 help the same URL without the %20X3 gets the current version
http://www.cakewalk.com/Documentation?product=SONAR&language=3&help=Playback.16.html
 

scook
Craig, in most cases you can delete the X3 and the current help will be found so when
http://www.cakewalk.com/Documentation?product=SONAR%20X3&language=3&help=Playback.16.html
points to the X3 help the same URL without the %20X3 gets the current version
http://www.cakewalk.com/Documentation?product=SONAR&language=3&help=Playback.16.html

Thanks Steve, good to know.