firstly, when you fill up and the pump clicks off as though it is full, give it a few seconds and try again. sometimes the force of the fuel going in makes the pump click off. if you still say you can't fill past half way i would be suspecting a fault with the fuel guage. it could be that you have a full tank but it's only reading half full. what would happen if you let the guage go down past the empty? if you kept a gallon can in the boot as an emergency reserve, you could see how much further you can go on 'empty'. good luck