This happens to me a lot, make 11 maps fine, then make another and the terrain just goes splat.
I can never remember what I did to fix them for some reason, but here is everything I try:
All on one layer,
Unclocked background layer,
All on background,
Merge and flatten,
Only letters in name,
No spaces in name,
No capitals in name,
Use an uncommon name,
513 by 513
512 by 512
Terrain grayscale,
Mask RGB,
.png file.
I'm afraid that's all I have, hope you manage to fix it.