Alecto farting

Happy birthday and congrats to the forum and members for year one! My .22 Alecto does not appreciate one pump for indoor shooting. Every few shots instead of a bang it it just gives a mouse fart. Anyone else come across this and what can be done about it? It hits very hard on two pumps and three pumps must be for big game and wears me out in no time.

If you adjusted the valve try backing it off just a little.

Thank you for the response Fastair. I have not adjusted anything other than the palm shelf.