Fillup issues

Here is my problem:
I buy these Arco gas cards from a local charity. They give me $50.00 per card. So I pull up to the Arco and fill up like normal. This usually leaves me with about $19.00 on the card. When I return to the station for the next fillup, where I will need about 9 or 10 gallons, I use whats left of the first card. Then I stop, put in the next card for $50.00, and try to fill the rest of the tank. But I cant. I cannot pump more than 1/2 a gallon into the tank. I ran off 460 miles on that tank, that should have been at least 9 gallons, and I only put in 6. The gauge reads full, but my mileage is all messed up and i KNOW the tank is not full.
Whats up?

Any ides will be helpful