Hmm. I've never had this problem before, but try re-exporting the maps and then dishing it out to others again. If that doesn't work, try changing the rendering systems from OpenGL to Direct3D9?
If that doesn't work either then you may just have to scrap the main map all together and copy or remake it and then re-join the maps together.
Sorry I couldn't have helped any more. ;-;
Good Luck
~Nym