Open FL Studio
Go to Options
Select Manage plugins
Click Find plugins
Make sure the correct plugin folders are added in Plugin search paths
Enable Verify plugins
Enable Rescan previously verified plugins if needed
Enable Rescan plugins with errors if needed
Click Start scan
Wait for the scan to finish
Open the plugin database
Right-click the plugin you want
Select Add to plugin database
Select Flag as favorite if desired
Check that the plugin format is supported by FL Studio
Install the plugin in the correct 32-bit or 64-bit version
Restart FL Studio after scanning
Update FL Studio to the latest version
Reinstall missing plugins if they still do not appear