By map masks, do you mean you created a custom terrain mask for your map? If so here are a few solutions that might help:
1. When you created your mask, did you make sure the colors were Red(255, 0, 0), Green (0, 255, 0) and Blue (0, 0, 255) exactly?
2. In your photo editing software did you make sure it was 513x513 and you set it to be RGB Color when you created the blank file?
3. Did you make sure it was saved as a png file and did you type in the name of your file correctly in the FeralHeart map maker?
And if it wasn't a custom terrain mask and you are just trying to select a single terrain from the FeralHeart map maker, then I would consider re-downloading FeralHeart, just to make sure that files weren't accidently removed at sometime.
If I can clear something up more for you just let me know