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, 
                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?

So I figured out a work-around quite a while ago. Since all of my objects were relatively similar (prefabs of approximately the same object), I was able to simply store their positions and values, delete the old objects, and create new ones in the same positions that did not have rigidbodies on them by default.