Moss places only the very front of her stride foot on the pitchers plate. It is common for her to lose contact and have her foot move an inch or so to the rear as she transfers her weight.
DD got warned for pretty much exactly that same thing last weekend in a 14U Tourney. The opposing coach at first base brought it up when she came in as relief. We now have pictures of it and sure enough, no argument she was losing that inch or so of contact at the rear of the rubber on some pitches. Just one more thing to work on now rather than later. Once it was brought up she never did it again in that game. That we saw. ;-)