Some of my VST Plug-ins don´t open although I can find the .dlls in the VST folder. Several scannings of the VST folder didn´t help... Other audio plug ins work but mostly my guitar plug ins (like guitar rig 3 or 4 and amplitube 3 or Ozone 3) fail to open. Simply nothing happens. Any ideas besides new installation? Thanks from Germany Stuff

Are they showing in the list of available plugs? 

You have to manually add them to the list Sonar scans on start. Just because you put them in the VST folder doesn't mean Sonar knows they are there. 

You must run a scan on them after you install them. Add them to the VST list in plug in manager and run the scan. Once Sonar knows there are there from a successful scan, it will pick them up and you can use them.