Underfilling is common. You can't really be sure how much CO2 they give you when you exchange the tank. One way to double check is to weigh the bottle when it is empty and weigh it again when it is full. Except you need to do that at the store to avoid a trip back if it is underfilled. You will certainly look like a picky bastard.
Best advice was already given - find a place that you are always happy with. Stick with it.
Also, in case you do not know, the high pressure gauge of the CO2 tank does not show a gradual decrease as the CO2 runs out. It will stay at say 800 forever then within a day or within a week will go down to 0. It basically tells you "I am full" and "I am empty" with very little room to react. Some folk have reported a sudden dump of the remaining CO2 - when it gets very low it messes up the regulator and it does not escape gradually but at once. That does not happen every time but it could happen.