I just wanted to make sure sidechain was working for you. The Sonitus Compressor and Gate bundle with SONAR also side chain. So do Fabfilter and other third party plug-ins I have experience with. I do not have any Waves plug-ins or Sonalksis compressors so cannot help with those particular plug-ins.

I have a couple Fabfilter plugs. EQ and comp, one comp is SC. It does not work in Platinum, but does in 8.5.

The VST3 Pro-C sidechain works fine for me. The Pro-C VST2 sc version also shows a sidechain in SONAR but I have not bothered with it.
 
 
 

Do you have 8.5 32-bit and Platinum 64-bit? It could explain the difference.
It is unlikely the reason, but have you checked that "Max used inputs" is set to 4 in Plug-In Properties?

Adq
Do you have 8.5 32-bit and Platinum 64-bit? It could explain the difference.
It is unlikely the reason, but have you checked that "Max used inputs" is set to 4 in Plug-In Properties?

Both are x64.
Checked in the plug in properties, Max used inputs was set to 2 on the C1, so I put it to 4, however I still do not see it as an option to sidechain into. One weird note, though, I see in the options C1 Gate, but I'm not even using that in the project. I deleted the C1 comp to see if maybe it was just referencing the wrong thing, but it stays in there. Something's f*cky.

Followup:
 
I just think something is f*cky with this project in general. I can sidechain to the built in bus compressor in pro channel if I create a new project and try it, but not in the project I'm working with. But in either scenario, I can't sidechain to Waves plugs. This might be two separate issues. Anyway, thanks for all the help. I don't think this thing will progress much beyond what was already said. I'll work with what I have and at some point maybe a redo of my entire system will sort it out. It's getting to be that time anyway.

..never mind, just reread the OP's post.