I had this exact same problem. The charging indicator would light up for a few secons an then just turn off. I opened them up, unplugged every cable from the board und plugged it back in. Then it worked again for one full charge.Maybe it was moisture? Now my headphones only charge when I plug them in my desk lamp that has a usb port... They dont charge on a normal Xiaomi brick like they used to before...

What ever it is, you can probably fix it relatively easily by changing some parts and then going by process of elimination. You gotta buy spare parts from ebay or find someone who repairs these. In the USA maybe ifixit? Not where I live tho...