If the egg is moving slow, then it is NOT debugged. If it is a fast hatching egg, then yes, it is debugged. Open up the back of your tamagotchi. Unscrew all 4 screws, lift up the reset button, use and eraser, erase that circle and then the DEBUG screw, and then re-assemlbe your tamagotchi.
Where did you purchase your tamagotchi? If it was just at a regular store, that would be weird, and it must have been a glitch. If it was bought on eBay, it is not that surprising. Sometimes buying tamagotchis on eBay is wasting your money- you never know if it is a fake, if it is ruined, etc.
"I can't... Didn't you read? I opened it already "
I doubt that would be a problem if the tama really isn't working well. I don't know about you, but we in my country have a rule which says the sellers must be open to change or take back anything for any reason (including silly things like "I don't like the colour") within 3 days. I see you opened the topic on the 14th, so that is already 3 days, but maybe working days don't count. There is some other rule which says they must change or pay the money back for things if it has some normal problem (and yours seems to be one), within 2 weeks. But yeah, you have to have the bill you have been given when you bought it.
I think you must have some rule which is at least similar to these. The best thing is to have a try in bringing it back I think, you cannot lose anything by giving it a try.
I hope there is some solution and you will succed in some way
If you cant fix it, and you can't bring it back, then you should report the problem or buy a new one. But if your tamagotchi's working fine then I don't think you should worry about it.