How are you determining how full the tank is? If it's the typical little LED bargraph display, check that the top sensing electrode connection is solid and the little wires unbroken and so forth. (Alternately, if the tank is translucent, you might be able to shine some light in and verify the water level.
If the tank truly isn't full or very nearly so, I would suspect a plugged or pinched or otherwise inoperative vent line. If you have trouble draining the tank when the fill port is capped (tightly), that would also suggest a bad vent.