I tried all the above for my UE Boom 3 - completely flat and wouldn't charge .It had the red light flashing when plugged into a good charger tried the on/off volume tricks ,USB in out 30 times even stuck it in the freezer but what worked was plugging it into my laptop USB- the white light on the UE boom at the charging port started flashing (it hadn't with the other methods) and after an hour it was charged - presumably a communication issue. So thank you for the advice !