I'm 100% sure now that headlights washers are equipped with individual pumps, 2x for headlights, 1x for windscreen
So as snapdragon mentioned, there will be like a very thin bactericidal growth of slime which got to the bottom of the tank where the filter is.
I've had same issue recently, headlight washers were working fine but the windscreen jets were literally dribbling.
So, driver side wheelarch near the driver's door, unscrew two bottom screws, pull the wheelarch with a flat scredriwer and you will see bottom of the tank and a pump.
Pull the pump up and there will be left a black rubber ring which is actually a filter.
If there is a water in the tank I will guarantee you water will shoot out as soon you pull this filter out. Clean of properly think skin type growth and put bits back together. But as mentioned before, give a quick.blast from the top ideally with a jetwasher or couple of bottles of hot water to flush the rest of the growth from inside the tank.
If you don't do that it may get block almost instantly or withing coupe of days, it happened to me.
Sorry for the write up but @ A&E atm very bored
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2006 D3 S8 5.2 V10
Ceramic Brakes, Bang&Olufsen, DTV, DAB, Rear Camera, BT, TPMS, Soft Close, Auto Boot, AMI , 4 Zone AC, Home Link, All Electric Blinds, Fingerprint, Ski Hatch, Black Extended Leather, Heated Sport Seats, Alacantra, Carbon Trim inc Aluminium Pack 2.
2015 A3 S line 1.4 TFSI - for the missus to keep her away from my car.
Standard S-line equipment, no extras.
2006 D3 A8 3.7 V8 - SOLD but not forgotten