Double check for the following with your file:
- 513x513 in size
- The Alpha Lock Channel is removed
- Flattened layers
- It's set to grayscale
- Saved your heightmap as (MapName)Terrain.png
Also check for double particles. Go to FeralHeart>media>particles and search for any .particle file ending with _0, _1, _2, _3, etc. Be sure to delete them if you have any.