It's the automatic fuel cut off for when the pump thinks the tank is full - if it detects any kind of back pressure the pump will stop fuel flow.

Some pumps are more fiddly than others and it's purely an angle thing - so you have to keep playing with trying different angles and the like. You'll certainly find that if you have people in the car the angle you need to put the nozzle in at will be different.

The only thing is to keep trying and it will sort itself out. I get this all the time with hire cars.