I had that problem with the extinguishers in my garage. After cleaning them out, I took a small piece of painters tape and stuck it to the end of the nozzle. Keeps the insects out and is easily blown off if you need to use it. Another area to check is the bottom of the extinguisher, wasps sometimes tend to build nests there and you don't see them until you use the extinguisher. Every month I pick mine up and turn them upside down. Turning it upside down helps to keep the powder inside from packing and you'll also spot a wasp nest if it's there.
