Problem with collision on heightmap

I experience some problems with my robot walking over my terrain genereted by a heightmap. When my robot starts walking the collision with the terrain seems to be working, but after around a meter the robot starts to fall through the terrain. If I move the robot around on the terrain it will have the same problem after a meter. I dont know why this is happening so I was wondering if anyone has solution. I have included some pictures and code that will help to understand my problem. I generated a heightmap(257*257) with c++:  I have used the heightmap provided by Gazebo as my starting and this is my heightmap.world file: 0 0 -9.810.0011000quick100-5 3 5 0 0.8 0model://sunmodel://dyret_autogentruemodel://heightmap/materials/textures/heightmap_test.png10 10 0.30 4.7 0falsefile://media/materials/textures/dirt_diffusespecular.pngfile://media/materials/textures/flat_normal.png1file://media/materials/textures/grass_diffusespecular.pngfile://media/materials/textures/flat_normal.png1file://media/materials/textures/fungus_diffusespecular.pngfile://media/materials/textures/flat_normal.png12545model://heightmap/materials/textures/heightmap_test.png10 10 0.30 4.7 0 And it looks like this when I open my world.launch file:  After a meter of walking it starts to fall through the terrain: