blender object passes through terrain but not objects

i have a object i imported as a fbx from blender. i cheaked generait coliders and added a rigedbody. it keeps falling through my terrain and i cant figure out why. so i tryed adding a box for the floor and it sits on top of it nicely.

i cant seem to finda awenser that works so far. i was hoping some one might know. it also passed through other blender objects... so i cant make terrain in blender eather

Don't use mesh colliders for moving objects.