Open Fl Studio, click "Options" and select "Manage Plugins". Next, click "Find Installed Plugins" on the top left of the Plugin Manager window. Make sure that both Rescan previously verified plugins
and Rescan plugins with errors options are checked, and that Combine VST and VST3 versions of a plugin is left unchecked.
Default Windows VST2/VST3 Plugin Paths:
VST2: C:\Program Files\VSTPlugins
VST3: C:\Program Files\Common Files\VST3
Here's how this process works on Mac:
If you have additional questions, please contact support here.