You may need to reverse the throttle channel on the transmitter.
You may need to decrease the "zero throttle" endpoint to arm the ESC properly. This is a common problem with the Castle Creations ESCs, but also happens occasionally with the Schulze ESCs as well.
Lose throttle control on a fixed pitch helicopter and it becomes impossible to bring down (heli flies away on its own).
This can happen if the heli ESC is set to "adjustable endpoint mode". When this mode is set, the heli ESC will treat the lowest throttle position it sees as the zero throttle. However, this can cause serious problems for a heli.
Imagine that your low throttle position is -100. A radio glitch occurs and the throttle ESC receives a fake -200 throttle position. Now, when your stick is at -100, this looks like mid-stick to the ESC, and you will be unable to turn the main motor off.