Rebuild lighting doesnt work on landscape

Hi folks, I have created a heightmap in a terrain generator call L3DT, The height map size is 256x256. I used the height map to create a terrain in Unreal engine. However I found the rebuilt lighting does not change any result, after I rebuild the light, the terrain still has the exactly same weird lighting
Here is the screenshot of the terrain after rebuild light, it is exactly the same result before I rebuild the lighting.
Can some one explain to me why would that happen? and how can i fix this issue?