Make sure the gas cap hasn't been replaced with a vented unit. The cap should be a non-vented cap, and venting is done through the vent line. Also possible to have a small cut in the refueling vent line, which is the small diameter line on the filler neck. Need to pull the tank to get a good look at it though...

New gas caps are like $5 at any parts store. Gasket in cap could be bad....