Why is my Morino tamagotchi no longer able to detect sound reliably?

The Morino tamagotchi has a predator function that requires an A button press followed by tapping on the shell to scare the predator. This uses what I believe is a listening device to register user input.

My tamagotchi was 50-50 on whether or not it would register any tapping on the shell before but now it seems that it no longer ‘hears’ anything.