RE: How to troubleshoot an electric problem?
First DON"T PANIC!
the 8.4 Volt rating is the NOMINAL voltage for the pack The voltage above this rating is what you are flying on. At 8.4 volts the pack will appear to be depleated.
Since you have charged voltage, sounds like a speed controller or motor problem. Does this system have an arming sequence you have to go through to start the motor?