2004 Dodge Truck gas filling

The only way to put gas in the truck is to trickle it in. My mechanic has had the filler hose off and it is unobstructed. He also probed the valve-like mechanism and it opened and closed freely. I did notice that the flapper, that opens and closes when the gas nozzle is inserted, is missing. I don't know if that could be part of the problem or not.