Bristol_Jonesey
Try doing a VST RESET followed by a RESCAN

I've done that several times, same results. I've removed vst folders from the scan, rescanned,  added the folders back scanned again. I've rolled back the Sonar versions, repeated all of the above steps and the same thing happens.
 
I've checked the XLN FAQ's as well and that was pretty useless.
 
It's very odd. Why would the scan see the dll, but not add it?

I had similar problems. I managed to get Addictive Keys working by unistalling, re-registering and installing the VSTis into new subfolders in the VST directories. I don't know which change made it work, but it did work.

Delete Waves SoundGrid Studio or StudioRack

It sounds like you may be using a custom plugin layout?

I finally got it working, but it will only show the 32 bit version, not the 64. It's not a big deal really, but still odd and a little irritating to the OCD. lol