Why is my cup size adjustment not working

I follow the recommended sequence for "calibrating" my Nespresso Inissia espresso and lungo buttons to the size cups I want but it doesn't work. I push and hold the button down, the pump comes on and coffee dispenses, but then the pump will stop without me lifting my finger off the button after only a couple of seconds. Next time I try to make coffee it may be a short pour again or it may keep going and going and would overflow unless I push the button again to stop it. Generally, really inconsistent dispense time. And before you ask, I always use the same Nespresso capsule, Ristretto, and not any imitation products.