Fuel Tank Capacity

[Driver]

I am having a problem with the fuel tank capacity. It is a 3920 Cyclone. All that I can get in is about 19 gallons in the tank. I am told and have read it holds about 26? I measured the tank and it comes to 36 gallons. The fuel neck was fixed because, of fueling slow. So, my question is why am I only getting 19 gallons in the tank.