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.