The last two times I have filled up with gas my car would not accept a full tank on 2007 Ford Mustang

10 gallons and then 13 gallons. What could be causing this?

Asked by for the 2007 Ford Mustang
Could be a vent control valve problem (emission control system) or could the tank be dented/damaged underneath?
1 more answer
That and it could also be the weather. whenever the pump stops give it a minute for the gas to settle in. I can fit an extra 1/2 a gallon when i do it with my '07 'stang