OnCollisionEnter not being called sometimes

I have a sphere with a RigidBody and sphere collider component that will not call when my thirdpersoncontroller is directly above it. My character jumps on top of the sphere and it will not call the OnCollisionEnter event.

It will do this event when the sphere moves beside it but not when I am above the sphere. I have tried to change the radius and height of both the sphere and the controller but to no avail. Any one else run into this problem?

I fixed it by going into the OnControllerColliderHit function of the CharacterController and doing a check for the rigidbodies that i want to test against.