It's a long time ago but this is what I remember

(1) Try clearing the nozzles with a needle. Sometimes in hard water areas you get a build up of chalk
(2) If that doesn't work you have to progressively clear the pipes. This depends on what you can get to. Sometimes there's a non return valve so you have to do it by stages and always blow towards the nozzle and disconnect the downstream end.(3) Disconnect at the nozzle and at the pump and blow through the pipes (airline or lungs)
(3) Remove the reservoir and flush out all the sludge
(4) Clean the filter in the reservoir.

Other problems:
different detergents can react with each other and form a sludge
Detergent and hard water can form a sludge
Algae can grow in the reservoir forming black sludge - bleach will kill it.

Hope this helps