You may have air in th system. if so you need to bleed the rads with a rad key or flat ended screw driver on each rad, do not remove the bleed screw just open it up till you hear the air coming out then followed by water if when you open the screw and water comes out this rad is ok. The system should run at around 2 bar, top up the system and bleed the rads to get the air out of each rad. Do not forget to top up system after each rad bleed.
Hope this helps
paul
December 2010