[BrianG]

First things first. Reset all your ESC settings to deault, update the ESC, and try it out.

If it still doesn't work, verify the throttle trim center and set both reverse and forward EPA to their max values. That will give the broadest possible signal with neutral perfectly centered. Then, calibrate radio throttle throws to the ESC.

If the ESC still doesn't respond correctly, reload the ESC firmware and reset all ESC settings to default values and recalibrate the ESC/radio.