Quote:

it fells through my terrain too:(


This could happen if you have a highly tesselated terrain and/or object. A quick solution is to set the object to PH_BOX/PH_SPHERE instead which generates less contacts and avoids contact misses.
Ideally your object should not be larger than 2x2 vertices of your terrain. So if terrain face size is 10x10 then your object should be smaller than 20x20.