First time this happened to me I washed drawer etc, but eventually paid for a call out (£65). The problem, as has been stated, is with the water supply through the plastic filter leading to the central fabric conditioner tray. I guess poking the build up from the holes from below would be a reasonable fix, but the 'engineer' actually pulled down the flap containing the dual rows of 2x10 feeder holes (it remains attached at the back but can flop down from the front) and removed the caked on grime from above & below with a toothbrush.
Two yrs later (today) I've just done it myself. The 'flap' is very tricky to pull down (I didn't see that bit of the paid-for operation), which is why I looked online here for guidance. I ended up partly screwing a screw into one of the feeder holes and gently using it to pull down the flap with pliers - a wire coat hanger may have been less stressful, but I didn't have one! Anyway, all is cleaned out & functional again - run an empty rinse cycle before your next wash.
You'd think the panel causing the blockage would have an easy-release, but maybe they hope for call-out charges? My fix-it man certainly said, 'Oh, you weren't supposed to see that', when I looked over his shoulder and mentioned how easy it should have been to fix oneself!