There are known issues when creating plugin thumbnails for certain third party plugins (Waves, MeldaProduction, etc.). These issues stem from the use of GUI's (e.g. OpenGL) which aren't supported by how FL Studio creates plugin thumbnails.
Note: This doesn't affect users with NVIDIA GPU's.
For users with dual graphics (Intel integrated graphics and NVIDIA graphics), if your NVIDIA GPU supports Optimus Technology there is a solution:
AMD & Intel Graphic
There currently isn't a solution for blank plugin thumbnails being created for third party plugins with GUI's which aren't support by how FL Studio creates thumbnails. We are looking into a possible solution to this problem.