The "ready" light comes on in electronic flashes when the capacitor is about 1/2 or 2/3'rds charged. Furthermore, the maximum voltage applied to the capacitor depends on how charged your batteries are; i.e., the voltage of the supply. So for consistency, I suggest (1) waiting at least one minute after the "ready" light lights up, and (2) powering the unit from AC instead of batteries.

Mark Overton