Beko 3731 dishwasher - clean bottom, dirty top!?

My dishwhaser (3 yr old) has started to spray and weld a green scum on glasses and cups in the top tray. Plates, cultery, pans etc in the lower tray are cleaned without any problem. Have taken the pipe feeds and water jets off, cleaned and replaced, tried a variety of salt/tablets/rinse aid products but still no resolution. Any ideas appreciated.