Hi, OK first check the charger, is it charging correctly please confirm this if possible by borrowing a charger, check the charging port, it is a loose fitting , do you always get a green light on the charger ?, if not check the connection at the charging port.
if after use say greater than 2 miles , the batteries charge up within a couple of hours, that points to your batteries losing the ability to take a charge, now first check the terminal connections to the batteries, are they clean and tight.
Check the brake mounted on the side of the motor, can you hear a click when powering backwards or forward , it may be advisable to get the wheels off the ground, please take care when doing this.
If the brake has become lazy it may not be releasing 100% , moving the wheels around when slight , (just so yo can hear the click, it may be very faint), power applied will test this, again please take care when doing this test
OK so the batteries are charging up, then sadly 95% sure it your batteries, that need replacing. the other 5% being a faulty controller, dodgy motor, but this is very rare.

Please leave feedback if the suggestions help.
Regards GeWiz.