It is possible for debris to get into the seats of the check in the back flow and water would drip out of the vent port of the backflow preventer. Water would not flow backwards to the domestic side. If you are losing pressure due and you see water coming out of the vent port than it is a sign there is debris in the check valve. If you don't see water coming from the vent port than it could be a leak within the system or a failed expansion tank.