This is actually kinda a common thing. The map maker can be so finicky when it comes to the creation of these heightmaps. When you were creating the map, did you use separate layers and then merge them together in the end? That can actually mess up a heightmap and there could possibly be a few ways around that ~ upload it somewhere and then re download it.
Another issue could be the alpha layer. I'm not sure how to remove it in Photoshop Elements (I use that also, and when I tried to make a map I was having the same issue to the point of where I just used a different service). There could be tutorials online on how to find the alpha layer to remove it.
Lastly, make sure the map name doesn't have any spaces in it. That can really mess with a file for some reason!