My '95 BMW is like that.
Fuel above the flap just sits there unless you depress the flap.
On most cars, you can slowly pour an add in, and it will trickle in pretty quickly even with the flap closed.
On the BMW, I took a bottle of injector cleaner to the gas station, and ended up having to dribble a bit in, and then open the flap with the pump nozzle to get it into the tank.
Rinse and repeat, until I finally got all of the stuff in.