A3 rear wash-wipe delay: cause?

My 2000 A3 used to spray water on the rear window as soon as I pushed the stalk forward - now I have to hold it forward for 5 secs, when I can hear
the power activating but no water immediately.
Any ideas as to the cause? The reservoir is totally full and I have cleared the nozzle with a pin. For what it's worth the front nozzles work fine. I can't see how a blocked water line would cause a delay, surely it would just not work at all?

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