Sounds like you have the motor routed to an improper motor pin. It could also be that the FC has a short on the pin you are connecting to.
Did you run ground wires to your ESCs? Try that.
If you have flight controller pins, try reflowing the pins, I’ve seen cold solder joints cause issues like this.
If that doesn’t work, try changing your mixing to move the motor output to a spare pin that works.
Hopefully that helps you.