I understand that on my X type the wipers are prevented from working in wash mode if the bottle is empty. I expect that the same feature is on S types.
The other thing that very occasionally happens on my X type is that the hose gets itself into a position such that a kink happens where it needs to bend when the bonnet is opened and closed (close to the bonnet hinge in my case) Have you checked to see if the washers work if the bonnet is left open?