Doorbell Charge

Hi there, I have a problem with my doorbell charging. I plug it into a micro USB port and charge. Within an hour, the blue circle on the front is fully illuminated indicating the bell is charged. However, when I take the USB cable out and mount the door bell on the back plate outside after re-connecting the doorbell to my WIFI, the app then indicates that the doorbell is not charged. I have tried charging overnight for many hours but the same happens. I have tried (multiple times) a factory reset to ensure the bell is connecting to the app correctly but still the doorbell doesnt seem to function. I have paid into a protection plan but with the doorbell not functioning, this is a pointless subscription. I am interested in upgrading to the peep hole doorbell but this is quite expensive so I would like to understand what is wrong with my current doorbell. Can you please help? Thanks, Rich