That's happened to me a few times. There's mutiple different things that it could be. Such as; your computer disapproves of a mesh, you just need to restart your computer and try again. I can't give a good diagnosis because I don't have enough info, but that's probably the most you could give due to not knowing the problem. But how I've fixed it in the past, is adding an object, saving the map, exiting, restart computer, go back into the game, remove the object that you placed, then export.
This may not be much help, but at least I could give you how I fixed that problem
