[David Pozzi]

Good comment James.
If you can't feel the powder move inside, it's compacted. There are lots of out of date extinguishers that get dumped every year, the service can't tag them because they are too old, but they can service them without tags for a non-commercial garage use. All they do is discharge it into a container, refill the powder back in, clean the valve, & pressurize with nitrogen.
Try & reduce the amount of flammable material in your garage, don't store a lot if fuel or paint thinners & keep them away from any welding or grinding. We had a tractor fire when a drop light fell into a bucket of thinner that was being used to clean a tractor. Nearly caught the barn on fire.