This is just a wild guess because I couldn't think of anything else, but is it possible that they got some moisture or some oil in the tank that could be causing the needle to stick? Try spraying just CO2 (no paint) at a piece of white paper and see if anything is coming out.
Another thought is that perhaps they overpressurized the tank. Is the supply pressure gauge reading about what it should for a full tank?
Just guesses. Sorry I don't have any real advice.