What brand and type of exthinguisher did you use?
The extinguisher also kept sissing slightly afterwards. Which basically means it was losing pressure, also something it shouldn't.
That's the reason they advise you to get it refilled and recertified after
every use. I't shouldn't do that, but just to be sure...
Something I do every 4/5 moths is to flip all the powder-exthinguisers upside down and tab the bottom and sides with a rubber-hammer to make sure the powder hasn't clotted (??) (and check the safety-pin,momometer,etc. Of course, you can't do that with a CO2 one. (you'll have to weigh that one)
And another thing: why use a powder-exthinguisher for 'practice'? It gives a terrible mess! (as you probably already found out) Why not a CO2 or foam?