Replace BUZ11 MOSFET with a TIP120
Without a schematic or basic wiring diagram, this is guesswork.
The saturation voltage of a TIP120 is not as low as the circuit voltage across the 40 milli-ohm on resistance of the BUZ11.  If this really is the reason, I'm surprised that the circuit is that picky.  Plus, I think the directions of the schematic are either incorrect or incomplete.
Do you have any idea what the BUZ drain and source are connected to inside?