FLStudio's plugin manager is not up to date with the latest versions of the SDK.
This was taken from the page "VST 3 Locations / Format" in the SDK documentation:
[quote]
On the Windows platform a VST 3 Plug ...
seems to be outdated, and when I try to do it the way as explained in FL knowledge base, it doesn't work either. Strangly enough the Trueverb gaveaway is recognized, but only that one. Anyone any idea?
Thanks for your time. ...