This could mean your buttons have dust underneith them, or perhaps your Tamagotchi is just old. In any case, it is normal, and perhapsa through clean would do you good. Simply unscrew the battery cover and the smaller screws inside to take the back off. The components inside are durable, but be careful to not break any of the small wires. Take the buttons off and rinse them in lukewarm water (THE BUTTONS ONLY), towel dry and place them back inside. Be sure they have no water left on them, however, as this can fry the tamagotchi's circutboard.
Hope I could help!!
~Tama-Nut~