terrain + mesh + mesh collider doesn't work?

I have a similar problem but using convex doesn't seem to work, at least not always.

I have a scenario where I have 3 object dropping onto a surface, the surface originally used a box collider and the 3 objects use mesh colliders pre-convex. The objects interacted with the surface fine but they didn't interact with each other so I enabled the convex option and immediately my objects started falling through the surface. I then changed the surface to use a mesh collider and one object collided with the surface but the others fell straight through despite each of the objects having exactly the same collider and rigid body properties (other than the actual mesh)

I am very confused and very frustrated as primitive colliders will not work with my scenario but I cannot get the mesh colliders to work.

Shouldn't a mesh collider with convex enabled also interact with primitive colliders?

I read somewhere that there is a limited number of faces the mesh can have. The meshes aren't particularly complicated but if there were too many faces how would the collider behave?

Are there any other properties belonging to an imported asset that might effect the collider?

@$$anonymous$$ieranSmart - it would be best to open this as a new question. You are far less likely to get an answer with a comment on a three-year-old question.

sorry I have, the link is: http://answers.unity3d.com/questions/815671/mesh-colliders-with-convex-option-enabled-doesnt-w.html