Hyundai Elantra wont take fuel

I have a 2001 Hyundai elantra the past few months I have been having trouble with the gas pump shutting off before my tank is full. The past week my car wont take much gas at all. Iit takes about 15 min to put in about 3 gallons of gas. I know the fuel tank is very low because my gas gauge is working and will go up and down when I do get fuel in. I have taken it to the mechanic and he is stumped as to the problem. He fixed 1 thing but when he took it to put gas in it he got 2 gallons in it then he had problems.

please help if you could.

thank you
trtgomez3