How can a Powermax combi boiler be repaired?

The system was drained to replace a faulty (leaking) radiator, and after two visits by Corgi registered plumbers they informed us that there was an airlock in the system and needed to be bled over and over again to remove air, despite several attempts,radiators come warm for a short time then go cold. Hot water to taps OK. Surely this cannot be right as when originally installed it would not take that long to commision. Any Ideas please?