All dry chemical extinguishers should be routinely inverted and tapped on their bottoms until the power inside is heard to break loose and 'whoosh' to the other end. Repeat this process a few times. This is called "fluffing the powder" and is extremely important on units that are subject to vibration.
The other issue is that the 2-1/2 pound, plastic headed piece of junk that is usually provided new should be replaced with at least a 5 pound ABC dry chemical. A 10 pounder is a lot better and you can actually fight some fire with it. As always, if you even use a one second burst from an extinguisher, you should have it serviced ASAP. This service removes powder residue from the valve and its associated galleries ensuring it will continue to hold pressure.
