The first thing I would check is your wiring. Make sure you have power and ground at the mainboard pins and then check for power and ground at the fan motors themselves. The wires that run to the hotend move every second that printer is printing. They fatigue and sometimes the wires break inside of the sheathing. But other times they get pulled on and things can get unplugged or loosened. If all else fails the BTT Octopus has 5v, 12v, and 24v fan outputs. 8 of them.