Why do you think the switch is OK? If it goes up but not down, it is most likely the switch or the wires between the switch and the
control box. One easy thing is to swap the switch wires at the
control box so the "up" switch is now connected to "down". These are small wires. Don't touch any big wires and turn off the breaker while you're changing things.
If it now goes down when you press the up switch, you need a new down switch. They're inexpensive and fail frequently. There's a lot of power around a
windlass so don't do this unless you feel comfortable.
If it still doesn't go down, then it's probably the control box and you need someone with some experience.
Carl