firstly have you checked for leaks?charge up system and go round all rads removing any air-do this a few times -when topped up check if water being discharged through overflow - the water has got to be going somewhere - make sure when charging system through fillling loop that you shut them off when complete.
Thats a first answer - there are probably others to follow from others but you need to advise boiler make/model to make it a bit easier
eddie
January 2010