Problem with collision/physics

This is my problem: I've a cube with BoxCollider and a rectangle/cube (Floor) with BoxCollider and MeshCollider.

Both are colliding when game starts (Cube uses gravity, tile does not)

The problem is: When the cube reach an empty space (With no floor), it doesn't fall in some cases. Sometimes it does.

It happens randomly and i don't know why.

You can see two gifs about my problem here: http://imgur.com/a/KioYh

Edit: I attached the gifs to thread

 

Im not sure how you have things setup (are you moving the floor and not the cube?), but try constantly calling WakeUp on your cube to see if that changes anything.
http://docs.unity3d.com/ScriptReference/Rigidbody.WakeUp.html

 

Check your object velocity in Y.