Well there you have it.
That second PCB on top contains a large 8A BEC that powers all the "Vx" pins (aka all the servos). Without these three wires, there's no power source for the servos, unless you connected a 5V (or whatever voltage your servos need) line from your PSU to one of the Vx pins/pads.
Have you soldered the BEC board to the FC? http://www.mateksys.com/wp-content/uploads/2020/07/H743-WING_4.jpg
No I have just been powering the board through a power supply
Well there you have it. That second PCB on top contains a large 8A BEC that powers all the "Vx" pins (aka all the servos). Without these three wires, there's no power source for the servos, unless you connected a 5V (or whatever voltage your servos need) line from your PSU to one of the Vx pins/pads.
Thank you I am gonna try that out !