Rads working upstairs but not downstairs. ?

Have all but one rads downstairs not heating and all the upstairs are heating. Bled all rads and only cold water coming out. Bathroom rad is one that you use a screwdriver on to bleed. And it gave out some air. But no water until I turned up the pressure on the boiler. Then it heated. But the downstairs rads are still freezing. Potterton combi boiler. Any suggestions welcome. Thank you.