Sounds like it is time for a new tablet. Batteries wear out and lose their ability to hold a charge.
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How to contact Amazon CS:
Use the HELP link at the bottom of any Amazon page, then NEED MORE HELP...then CONTACT US...and CONTACT US again...select CALL or CHAT. The Call option allows you to input your phone number, and they will call you. Don't use email, and don't Google for a phone number, the results are scam numbers.
I concur with Meya, time to retire that one. As to why some still work and some don't: That's normal variations in manufacturing and use.
Plus, if you actually have an "original" Kindle, those had user replaceable batteries so it (the battery) might not be original. I'll bet a bit surprised if you actually have a first gen model as that was a long time ago. Wedge shaped? Round charging cable not USB? Image below:
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