My DD did this from the beginning. It wasn't a big deal until she got teeth on both the top and bottom. At that point I realized it needed to stop. What I did was as I was guiding the breast into her mouth I used my index finger (of the hand holding the breast) to hold her chin down. It was just enough to stop that initial chomp. I did this for a while and then the behavior stopped. DD was only probably 6-7 months at the time, so I couldn't really explain to her. Maybe at 15 months you could use some words, but I definitely think it helps to slow that jaw down on the initial latch. Hope that helps and good luck!