Make sure the valves at both ends of the rad you will be trying to clear are wide open, it might also be worth seeing if there is a flow control on the pump which you can turn up to a higher speed, most pumps have this feature.
If you still have no joy.... leave 1 valve closed and draw a couple of pints of water through the air vent on the rad, then close that valve, open the other and draw another couple of pints... you will hear air bubbles in the rad if it is indeed an airlock.
Pete
January 2007