Fuel tank does not vent when filling

What can be done with a fuel tank that does not vent when filling? I have a 1999 Subaru Legacy Outback Wagon and my mechanic can not figure out why the tank does not vent. When putting fuel in the tank, the nozzle keeps on shutting off. I takes a long time to fill the tank! My mechanic said that to investigate further, he would have to remove the differential in order to drop the fuel tank. Anyone with experience with this?

Most fuel tanks have a vent tube along side the filler tube. Yours MAY be plugged.

Has it always don it?  Are you or a prior owner been in the habit of topping off the tank when you fill it up?