It could be one of these problems: you have too many maps you have downloaded from rps and such, you just downloaded too many meshes recently, your heightmap could be not made properly. Or you're missing the certain plug-in for Map Maker. I've had the problem before. What I had to do if none of these worked, was to re-install FH. If it STILL persists, your computer may be the issue, which is highly unlikely. Hope this helped. ^^