Powering a Servo with 12V Battery and Buck Converter

I wanna power my servo with the 12V terminal of a Mastercraft Nautilus battery pack (see here)(long story). The servo is a actuonix pq-12 micro linear servo (see here). I’m stepping the voltage down to 5V using a Yeeco DC-DC Buck Converter(see here). I’m controlling the servo with a raspberry pi 4b. Attached is the python code.. When I attach VCC and GRD to the PI the servo works perfect. 1 Full extension/retraction. When I attach VCC and GRD to the output terminals of the buck converter…nothing.
Note: Control(PWM output) is connected to GPIO17 in both cases.
Speaking of the Buck output, they’re the right voltage. I’ve hooked them up to a little LCD circuit. Sure enough, the Buck converter will spit out some current.
I have a couple of guesses so humor me:
(a) VCC is in fact the PMW signal. Not the "control" wire connected to GPIO17
(b) The buck converter has a max output current of 5A. Maybe when prompted by control the resistance of the circuit becomes so low that it demands a current>5A. I tried adding a 220ohm resistor to test this theory and no marbles.

Thanks for any help you’re willing to provide

EDIT: It works now. My father suggested the problem was to do with the absence of a common ground. I connected the out- and GND pin of the PI, and GND of the servo together. Then I connected out+ of the buck converter to VCC of the Servo and it worked. I am confused why though. The voltage difference between out- and GND(pi) while unconnected was very small, 0.2milli volts. Can anyone explain to me why this alteration was crucial?

Thanks again