I think the vent is electrically controlled. If you push the flapper in a magnetic switch closes and the vent valve at the tank opens. Porsche has a unit up near the filler opening they call a vent valve. This is a picture of mine removed from the car.
If this unit isn't working you'll have trouble filling. Also the magnetic switch. A new one comes with a new vent valve.
If you can get the tank to fill by depressing the little tab where the side of the filler cap presses down when you thread it back down, then the magnetic switch could be the problem. That's what happened to me when I replaced the valve in the picture and didn't put the switch back where it's supposed to go, which is actually a recess on the side of the plastic valve. I couldn't get a half a gallon of gas to go into the tank. It worked before I took it apart so I knew it was something I did and was able to figure out what I'd done wrong. You might have a different problem than I did. I actually have an extra magnetic switch if you need it.
I had all the charcoal stuff apart at the same time and I couldn't blow through any of the lines that went back into the tank.