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 ...
Fl 11 and 12 and 20 are registered however my vst's are still in demo mode i think i may have dragged another fl file containing the same vsts that fl comes with in the library and somehow rewritten ...