Wheel colliders sometimes falling through the road meshcollider

Hi, i have a vehicle with wheelcolliders, i got it from asset, it works correct with native road in scene in this asset, road has a mesh collider, i created my own city with difficult shape roads and created more simple mesh for road collider, approximately 30 poligons, on horizontal road car ride well, but where road descent, for example, my wheel colliders falling through the road by a half and car cant ride anymore... i cheked my road collider anytimes and cant understand why it happens, it hasn't any holes ... amd i read that for road use cube colliders better than mesh, but it very difficult and long time to create many cube colliders for whole city with many kind of roads. thank you for answers..

P.s. sorry for my terrible english ) its not my native english