Mesh collider not detecting ground

Why mesh colliders in Unity 5 does not always detect collision with ground?

Sample setup: A plane, a mesh with Rigidbody and collider (mesh, box or capsule) not set to be a trigger, and positioned at some height. If height is greater than 1 meter, the object falls through the plane, but if height is less than a meter, sometimes it falls and sometimes it doesn’t. Same setup works 100% in Unity 4. Using a 1 meter thick cube as ground solves the issue, but doesn’t seem optimal as 5 faces of the cube will remain unseen by the player… This also means that you’ll have to replace all planes with solids (walls, doors, windows and maybe even other GameObjects) just to ensure things don’t get teleported.

follow these steps.

1.if you are not making plane as rigid body only using mesh collide then give some thickness to plane to avoid this problems its work fine.

2.if you are using mesh collide and rigid body on plane then you don’t need to give thickness Just keep on both the option(Use Gravity,and Is Kinematic) to avoid collision problems.

but I have also check this in maya created plane and apply mesh collide in unity 5.collision is working fine without any problems. but you can use above option to solve this problems .