either a blocked cold feed (usually at the tee piece/air separator joined part of the primary system) - or a bad airlock. as suggested you could try filling up the system from bottom to top using a hose pipe.. but you will need someone inthe loft too!
Hi
Please offer me some of your time/expertise in my following learnings if you may. Feel free to correct anything I have written that is not 100% correct and add any further parts if you wish to, to help me:
fresh AIR into a radiator/primary system (brought in with fresh water) is bad...