OnCollisionEnter does not fire (OnTriggerEnter does neither)

I already read through most of the questions and forum post, but no solution to this common problems seems to apply to my case:
I have a Blob object with a rigidbody2d. The blob has some child objects which contains box colliders. I use the merged colliders to collide the blob.
The is falling onto a kinematic ground (with rigidbody and box collider) and is also colling with another blob. The collision is handled by unity (the object does not pass through other objects) but neither OnCollisionEnter nor OnTriggerEnter were fired.

Any help on this problem?

Well, firstly, you don’t need to put a rigidbody on your static colliding objects, such as ground. If it’s not going to be physically moving, you don’t need a rigidbody, just a collider.

Now, how are you calling the code? You say Rigidbody2D, is the ground 2D as well? If so, you need to use OnCollisionEnter2D and OnTriggerEnter2D.

Most of the physics functions, and many other functions, as of Unity 4.3, now have a 2D variant, meant for use with the 2D system. So if somethings not working, it’s often you’re not using the 2D version of that function.