Central heating problem following new boiler ect?

Had new boiler, downstairs radiators and water pump also new valves on existing upstairs radiator fitted.However are getting an air lock in in the return pipe which means the new radiators downstairs are not getting hot only the pipes to the radiators. We have had the plumber back and he has flushed the system through. All was working fine for a day and we are now back to cold downstairs radiators.We feel we now need to go for a second opinion which is going to mean more expense!Thought I would appeal to you good people. Thank you