So I'm trying to replace a synth on an old project.  I right click on the little midi image in the track, choose replace synth, click on the synth...absolutely nothing happens.  Oh here's the latest:  the effects wont work on the master bus.

It would help if there was some info about the synths. Are they both VST?, any DX stuff?, both 32 or 64 bit? do they require bitbridge? A lot of things could cause this option to fail.
 
I never use the feature, but I'm sure lots of people do.

I've tried ql spaces on the master bus...nothing.  No bitbridge.  I tried to insert an instance of kontakt where play was..nothing.  Just non-responsive.  I haven't changed anything on my computer.  So this is bizarre.

I'm just going to re-install sonar.  I have a whole pro tools 12 installed...not even sure why I bother with sonar.

Happy Camper!  the Reinstall worked.  Luckily they've streamlined that with the command center and it only took a couple minutes.

Re-installing should be a last resort. In many cases where re-installs "solved" a problem, it was because some preference was reset, or some folder that hadn't been scanned was re-scanned. While it's great you got it working, if you don't find out why there was a problem, you may run into it again.
 
I have not had to re-install SONAR since I first installed Platinum, and given that I have over 2,100 plug-ins (including Play and Kontakt), I'm pretty sure if there was some SONAR-based issue involving plug-ins I would have run into it by now.

Craig,
 
thanks for chirping in.  You're a guru, you have to admit that Sonar isn't idiot proof and I'm an idiot.  I love sonar when it's working.

eph221
Craig,
 
thanks for chirping in.  You're a guru, you have to admit that Sonar isn't idiot proof and I'm an idiot.  I love sonar when it's working.

I disagree that you're an idiot, but I agree that not only SONAR, but Windows, computers, and other software that interacts with SONAR aren't idiot-proof. I have no idea why my setup is so incredibly rock solid, but if I could figure it out, I would bottle it and sell it.
 
I will say my life changed for the better when I got rid of those drivers that various graphics card installed. If it's any consolation, I often have things happen where I'm sure SONAR has a problem but then I find out it was pilot error...and believe me, I know this program quite well. 

Ok,  an hour later it's doing it again.  I insert a vst effect and the signal signal passes through the vst without affecting  the signal.

Is the FX button blue in the Mix Module?