Have you listened to the motor or tried to manually push the thruster down when operating the switch? If the motor is working but the thruster is not going down the wire strip should be slack which might indicate something mechanical jamming the drive tube. If the wire is not moving then it is a motor problem. The light and buzzer working when ‘up’ is engaged has no significance. It would always do that in a he up position.
I have had issues with the sail switches when they worked one way and not the other - slightly burnt contacts. You could test by swapping the wires for up down and see if it does descend. If so you can remove the switch and clean the contacts. It is easy to dismantle and clean.
I think a failing motor might have given you a bit more notice!