Take the gauge off the tank. Crack the tank open, listen for escaping high pressure air. If you have gas in the tank, is the tank on a timer? Is the timer on, or off when you check level? Using a regulator? Pinched line?

 The tank is least likely to be the problem. Either it has gas in it, or it doesn't.

My first guess is the meter. Any way to calibrate it? 

A pic or two of the CO2 setup might help, too.