Washing Machine (bosch)

It's caused by blockages the holes in the water divertor (or whatever it's called) in the roof of the soap dispenser. Not enough water flows through during the cycle to cause the siphoning action to happen which then flushes the softener through. I've noticed these holes regularly get blocked with mildew (black staining) since water lays on the plastic providing an ideal environment for mildew . A quick solution is to clear the holes using an old toothbrush or similar. Longer term is to carefully remove the water diverter panel by pulling down at the back and also sliding a thin metal blade (eg old knife) between sides of the panel to unclip it as you pull down. Rather fiddly and danger of splitting the plastic. However if you succeed you can then clean all the mildew away and replace. However it will return unless you manage to keep the soap dispenser dry - perhaps by leaving drawer open. I suspect this is a design problem since it never happened with an old Hotpoint machine I had.