Hmm, I seem to remember that 2 people had exactly the same problem on the way to Donny South once. Arm had lifted off the spindle I think. Push it down hard and see if it works after that.
There was a connecting rod between OS and NS wipers. In each end were plastic bushes which wear and become loose.
When they are worn they have a tendancy to drop off.
As above. Both myself and Rich turned up at the final meet point for Donnysouth a couple of years back. Both of us with ink black Bravs with failing wipers on the way. All very bizarre.