2D Compound colliders fall through objects.

I'm trying to make a compound collider out of a pre-existing group of objects in the scene. I do this by parenting all the objects with collider2ds on them under another object that has a rigidbody2D component attached to it. This seems to work when I group a bunch of blocks without rigidbodies already attached, but they fall through the ground when I group blocks that already have rigidbodies. Here's the code I'm using to group blocks:

 GameObject blockHolder = (GameObject)GameObject.Instantiate(blockHolderPrefab, 
                                                                         blocks.FirstOrDefault().transform.position, 
                                                                         Quaternion.identity);
                 foreach(GameObject block in blocks)
                 {
                     if (block.rigidbody2D != null)
                         Rigidbody2D.Destroy(block.rigidbody2D);//block.rigidbody2D.isKinematic = true;//.collisionDetectionMode = CollisionDetectionMode2D.None;
                     Debug.Log("block pos" + block.transform.position);
                     block.transform.parent = blockHolder.transform;
                 }

What's confusing me is that the two groups of blocks - those with rigidbodies to start and those without - look exactly the same in the inspector. What am I missing here?