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.