Well,
if it is indeed a downloaded custom map, I suggest you try uninstalling and re-installing it. Perhaps you simply installed it wrong- i.e. accidentally put the files in a folder they don't go in.
Sometimes downloadable maps have a readme file with instructions on installation included, so you could check for that.
This is, of course, assuming it is a custom map.
Now, if it's the FH default maps you are having trouble with, again, I suggest trying to uninstall and re-install the game and see if that works.
Lemme know how things go, love.