It is possible that the plug-ins has not been scanned correctly.

To force a re-scan of the plug-ins do the following:

Windows and Mac

• Go to Options > Preferences > VST

• Set your VST Plug-ins path, and click "Re-scan" or "Clear cache/re-scan". Audio Unit plug-ins (Mac Only) are scanned by default on launch.

• Reboot Reaper and your plug-ins will be re-scanned. Confirm you now have the plug-in correctly listed in your FX List.