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 12.5.59 [b:3dkypsax]32bit[/b:3dkypsax] does not find any VST plugin when opening a project.
It looks like it happens with any project made on my previous PC (So all projects except my last).
The VST's are installed ...