How much of the nozzle are you trying to cram in?
There's probably nothing wrong with it at all, you're just assuming that it should go in further than it does.
Years ago the nozzles used to be able to go right up to the hilt, but things change, and this isn't the case anymore.
They've probably done this intentionally to stop people jamming the pumps on and walking away.
...also, if less of the nozzle can fit inside the tank, the tank can ultimately hold more fuel before the pump switches off.
Good for the fuel companies who can make an extra few bucks, not so good for the people who are driving around wondering why their car smells like petrol because the tank is over full.
Open your fuel flap, take off the cap, get a tape measure, put the tape measure across the fuel filler, take a photo.