It was working fine until just after it reached its perfect care character stage, don't remember the name and then my Generation 1 Tamagotchi's A button stopped working and I have not been super hard on it and the other one I bought at the same time still works with no issues right now. I reset it once, and the button sadly did not resume working so I had to have one of my parents remove the battery since I could not care for another baby Tamagotchi since I'm not good with opening up small objects with small screws in them.
Target.com does not need them returned to be replaced, and at no charge so that was nice and I got a new one about a week ago which seems to be working just fine. I will re-use the battery I saved though, since it should have most of its' power and it's more environmentally friendly to use it until it dies before using a new battery in my other Tamagotchi that's been running longer. In some good news, I found 2 unopened batteries that are exactly the right size for Tamagotchi at home so that is really great!
I sure hope you can resolve your problem with your Tamagotchi! Good luck with that!