Hey there. I think I know the problem. You said you already exported your map, correct? And you went back to Map Maker and made some changes and attempted to export again. I think I know the problem. You cannot export the same map twice. But there's an easy solution to your problem. :) Follow the steps below to fix it.
((SIDE NOTE: Make sure while doing the steps below that the Feral-Heart game is NOT running!!))
1. Go to Start>My Computer>Local Disc C:>Feral-Heart>Exports
2. Now that you're in the Exports folder, look for a .FHM file that says the name of the map your previously exported. ((Ex. Say you named your map ETH, what you do is scroll down the list and look for a file that says, "ETH.fhm"))
3. Once you found the .fhm map you exported, right click on it and select "delete". Delete that Export.
4. Now, exit out of those windows and open up the Feral-Heart game. Go to Map Maker and go to the map you updated and now try to Export it once more.
I think that should've fixed the problem. If not, be sure to contact a staff member or something and ask them for further guidance. I hope this helped! Lemme know if it did!