I realise this is an old post, but just in case...
I had a similar problem with Eventide Quadravox not showing (I'm on Windows 10, Studio one 6.5).
The plugin shows in the Plugin manager, but when opening an old project that uses it, the plugin isn't found, and if I try to insert it onto a track, nothing happens. It works normally in e.g. Fl Studio.
I fixed it by removing all references to the plugin from Plugins-en.settings (which on Window is found in C:\Users\username\Appdata\Roaming\Presonus\Studio One 6\x64\
I believe you need to have S1 closed while doing this. Also, I made a copy of the original .settings file just in case.
Then do a rescan