Maybe the charging cord has a problem. But if you are using it for a longer time then you can switch it for a new one. I use Mi 4 band it's cheap and working well. I got it 3 years ago
@Stereoguy I had a similar issue with one of my devices is all. Would show connected and then either wouldn’t keep it or just wouldn’t charge fully. Got a new charger and it’s fine now. Not saying it’s not simply the device, just throwing it out there that sometimes it’s the charger that goes.