Colliders work half of the time....?

I have a platform made from a cube. Default colliders etc, with a texture thrown on it to make it look nice.(Box Collider) I imported a cylinder shape from blender (think a coin shape), since I couldn't apply texture nicely to the unity primitive nicely. I tried both allowing Unity to apply the collider for me (via the import checkbox) and applying my own mesh collider and rigidbody.

If I drop 50 coins on the surface, half of them will land on the surface, and the other half will make it through the surface and get stuck half way through, seemingly "hanging" off the top half of the surface.

Oversizing the box collider of the platform makes no difference, nor thickening the platform cube itself. I'm thinking it has something to do with the mesh I imported from blender, but I have tried so many different things to remedy the situation, my head is swimming.

Any suggestions?

Craig.

Make sure the mesh is centered around the origin in Blender, try using mesh collider again but make sure convex checkbox is ticked. If they get 'stuck' inside the box I would say the collider is wrong. If element is too small or speed too high normally it just falls through. If you have an object selected in scene you see green lines representing the collider, check if it matches the object well.

If you're using physics you may need to employ the raycast trick from Eric and Daniel. http://forum.unity3d.com/threads/6109-Raycasting-For-A-Fast-Moving-Ball