There's a piece on the charcoal canister they call the "on-board recover ORVR valve", or a fill check valve. This is a one-way check valve that will direct displaced vapors to the canister as you fill the tank. When this thing gets stuck, the vapors in the tank won't displace, and the nozzle shuts off because of the pressure increase. The part isn't too expensive and it's fairly easy to replace and not an uncommon occurence.