The mains problems with tamagotchis are :

• oxidized pine (if you forgot the battery in it for 20 years ago… or buy it new. usually that’s not a problem with bandaï ones, but with odd pets) you can fix it by remove the rust with a q-tip with vinegar
• bad contact from screen or battery . make sure the more screw you still have are well screwed.

Hope it will help someone