Did you put all your map files in the right place and name them correctly?
Or maybe check and make sure that your height map follows all of the rules.
1. It must be saved into a PNG format
2. It can only consist of 1 layer!(If you use more the game will crash upon importing it) Make sure you flatten/merge the layers on the image before saving it.
3. The image must be saved/edited in grayscale mode and cannot consist of or use colours(If not the game will crash)
4. The Image size should be even on both size, the standard size is 513 x 513 pixels.
5. The use of "Alpha Channel" should be left unmarked/turned off.
Thats unusual, Are you sure it isn't? Sometimes they can get lost in the exports file if you have alot a files in there. Trust me, I know. x-x
Yea, I'm sure. When I export a map, I usually drag my exports into a separate folder on my desktop until it's done.