When I add gas to the tank the pump keeps shutting off like the tank is full. I know the tank is less than half full. Is it possible that the filler pipe has collapsed or is there a vent that could be clogged or what else could it be?

There is an solenoid-activated vent valve above the vapor recovery canister uner the right rearcorner of the car; replace the valve and you should be good to go.

That is pretty common.

 

Those of us who worked at filling stations 40 years ago discovered this just happens with some cars. After thousands of cars it finally made sense why one would do fine and the next didn't. Fuel nozzles tend to foam up the flow so that the bubbles rise up the flller pipe ahead of the liquid fuel  to shut off the nozzle. It keeps liquid fuel from spilling out the tube. I learned to retract the filler nozzle from fully inserted back an inch or so, and it eliminates the kick off.

 

My Forester does just fine, the wife's van has to be back an inch and shooting fuel at the side of the fill pipe to defoam the flow filling the tank. It's an artistry of finesse as you discover the right tilt to get full flow.

 

What has contributed to the problem now is anti-theft designs with obtuse turns or baffles to prevent siphoning. There is also the variance between pump pressures from station to station, and even the different nozzles on the same pump. No two seem to be identical, but they all foam the flow to get the nozzle to shut off. 

My MIL's 2010 Forester just had this happen. The dealership charged her $110 or so so to clean some valve. I assume the fill valve but, I dunno where it is or how it's accessed. her car gets extremely low use (7k miles!) and sits for long periods of time.