[Tannaton]

As said if the washer bottle gets below a certain level (but not empty) - the headlamp washers stop working.

Also as has been suggested they only work 1 in every 5 times that the windscreen washers are operated and only when the headlight switch is in position II (or Auto and the headlights are full on).

I *think* they also wont work if the car is stationary also (to prevent you soaking cyclists at traffic lights) though I don't recall where I read that.

Some people do disable them by pulling the fuse as they can use a lot of water, but in many circumstances this is an MOT fail now.