My axolotl used to poop daily, a few hours after I fed him. If you don't notice your axolotl passing waste, it could be for several reasons:
-It might have broken down and sunk into the substrate. The best way to negate this is by stirring up the substrate during tank cleaning so you can suck up any little poop particles that settled in.
-The axolotl might have eaten it. Gross, I know, but that's nature, and axolotls are just little animals.
-There could be impaction; unlikely, but possible. Depending on the morph, and the size and color of the substrate, this may be visible from the outside.