Collision Detection Problems

Hello fellow developers.

My game consists of a base building mechanic i am trying to code however is proving difficult to do. I wish to 'snap' the foundations together through the use of collisions. I have done this by making a 'Snap Block' which has a collider on it. When i instantiate the new foundation i have it check it's collision tag through the use of:

  OnCollisionEnter (col : Collision) {
     if (col.gameObject.tag == "SnapObject") {
     //Some code to teleport the foundation to the snap block and freeze its position.. (it works ive tested it)
     }
     }

This works perfectly fine until i child the 'Snap Block' to the foundation. Can anyone tell me why this happening and possibly how i could fix it?