Tonight I started using frozen ducks with my dog. I started by just giving the duck to my dog and commanding hold. He had an issue with wanting to chomp down on the duck. I commanded hold, gave a nick with the collar, and hold again. This seemed to help. A couple of times when I went to get the duck and commanded leave it he chomped down on the duck again. What do I do to correct the chomping? This is not an issue with bumpers. Is this a sign of a hard mouth?