Let me just start by saying I've been using Sonar since 7. I made the jump from 8.5 all the way to Platinum somewhat recently and one of the big selling points for me was VST3 and the related support for sidechaining Waves plugs that comes along with that. Anyhoo...
 
I'm working with a project in Platinum and I need to be able to compress a track based on a sidechain input. I add C1 SC onto the track to be compressed, set the output of my key track to... wait, C1 doesn't show up in the list of available compressor inputs. Try a different one, C6 SC. Ok, that one shows up, but my key track doesn't actually trigger anything in the compressor when I input it into the sidechain. So, knowing the drill, I load the Sonalksis sidechain comp into it just needing to get this done... doesn't show up in the list of available compressor inputs. I can go back to 8.5 and sidechain into the Sonalksis comp all day long. 
 
I messed around with VST settings in Platinum to no avail. Migrate to VST 3 and not. Made sure my plugin layout has the VST3 Waves plugs. Disabled the migrate features and reimported the Sonalksis SC comp from the VST folder, no dice. My key track is solo'd as is the track to be compressed, where any variety of sidechain comp can be inserted, and if I'm lucky, one or two will show up in the list to assign the output of the key track, while not actually doing anything. 
 
Am I the only one having this issue? 
I looked around and found a few threads relative to this, but their issue was basically not knowing how to sidechain or having issues that were relevant for earlier releases of Sonar. Am I missing an update or something? 
 
Any help is appreciated. It will take a very long time for me to manually reduce the snare volume of the overheads I need to process. Thanks in advance for any direction.

And it has just dawned on me that I'm going to sidechain the overheads in 8.5. 
 
But still, I'd love to be able to do this in Platinum going forward. ;)

Did you do a reset and rescan of plug-ins recently? I realize I say this to the point where it may seem silly, but so many unusual plug-in problems get solved when you do this. Not saying this is a sure thing, but it can't hurt, and may help. I do sidechaining all the time in Platinum so I know it's possible.

Anderton
Did you do a reset and rescan of plug-ins recently? I realize I say this to the point where it may seem silly, but so many unusual plug-in problems get solved when you do this. Not saying this is a sure thing, but it can't hurt, and may help. I do sidechaining all the time in Platinum so I know it's possible.

Yea, I have it set to scan every time I open Sonar. But I also manually scanned with each change made.

Automatic and manual scan is not the same as a VST reset. Here are the instructions for a reset http://www.cakewalk.com/Support/Knowledge-Base/2007013315/SONAR-X3-Full-VST-Rescan-Recommended-for-Upgraders
 
Do the bundled plug-ins with sidechain capability work?

scook
Automatic and manual scan is not the same as a VST reset. Here are the instructions for a reset
 
Do the bundled plug-ins with sidechain capability work?

Ok I have not tried the reset method. Doing now. Taking a while, many plugins. 
As far as the bundled plugs, I haven't really delved as deep into Platinum yet as I have in 8.5. Are you referring to stuff like the Sonalksis plugs, or something else? I'm still unsure if I've downloaded and installed everything that comes with Platinum. I activated the Melodyne stuff, etc. but I don't know if I missed anything. I was drunk when I upgraded. :/

WeekendAstronauts
 I was drunk when I upgraded. :/

Please provide steps to reproduce. I'm very keen to validate this.

Not Sonalksis but there are a couple of Sonitus plug-ins bundled with SONAR which sidechain. My recollection of 8.5, sidechain works the same with those plug-ins in the new SONAR as it did in 8.5. Here is a video describing sidechaining in the X series  (which still applies to the new SONAR) using the PC4K S-type bus compressor for ducking https://youtu.be/tkWeQR4UJf0?t=2m15s

Doktor Avalanche

WeekendAstronauts
 I was drunk when I upgraded. :/

Please provide steps to reproduce. I'm very keen to validate this.

As a Sysadmin at a software company, I lol'd. 

scook
Not Sonalksis but there are a couple of Sonitus plug-ins bundled with SONAR which sidechain. My recollection of 8.5, sidechain works the same with those plug-ins in the new SONAR as it did in 8.5. Here is a video describing sidechaining in the X series  (which still applies to the new SONAR) using the PC4K S-type bus compressor for ducking

Ok using the bus comp in the video, I can sidechain. So what does this confirm? I can't sidechain with third party plugs, but it's a go for the pro channel plugs that are sidechain capable.