On three occasions in the recent past I've come across situations where devices with lithium batteries, that are charged via mini-usb plug/socket connections, stop charging.
I had thought that this was due to either failure of the charging system or maybe the battery ageing but given that I've been cleaning up various electrical connections recently with Deoxit, I picked up a spray can of contact cleaner and gave both the plug and socket of a couple of these devices a good sluicing out and bingo, back in action.
I suspect that just as car battery terminals tend to oxidize [partly] due to their constant electrical potential, these micro connectors probably do the same.
Worth a try ...