Just had an extension built and added 4 new radiators. Plumbed them all in but they are not heating up. I have a single looped system in the house all ready and joined the rads onto that system. The pipes going into the first rad is hot and so is the second rad. Not getting any heat to the third or fourth or the one in the old part of the house that I joined them to. I have a worcester 28si combi boiler so it is big enough to run all the rads. Any help PLEASE!!!!!!!!