Likely dirty/corroded charger connecting points that make the charging contact intermittent.
Clean them, scrape off the layer of crust/corrosion with a jewler's screwdriver if you have to. Once they're clean and charge well again, I recommend putting a little dab of dielectric grease to protect them from corroding again.
Mine started doing that a year after I bought them and I was about to throw them away when I tried doing that and it completely fixed it.
Remember that Stargate SG1 episode where they find themselves into a huge underground facility that is under attack by external forces. Jack O'Neill accepts to pilot one of their drones to help defend the complex from a wave of bombers and is infuriated to learn that the attacker's bombers were manned while he believed they were unmanned.
spoiler
At the end it turns out the facility was actively poisoning the entire planet, threatening to wipe out all life and the "attackers" were simply the people of the rest of the world trying to stop it.That is what the Amazon data center is to me.