Two upstairs radiators not heating up

[luvdiy]

All radiators working perfectly until recently when I relocated a radiator in a downstairs room. Out of 10 radiators in the 2 storey house (5 upstairs and 5 downstairs) all are still working perfectly except for 2 in seperate upstairs rooms. I've bled these and the radiators did heat up. But when I turned on the heating system next day these same 2 radiators did not get warm. Both water pipes into each remain cold until I bleed the radiators and this allows one pipe only to get hot in each case. The other pipe in each does not warm up. So for some reason the water is not circulating in these two radiators.
Why is this? What can I do to solve the problem.
Any suggestions welcome.

Thanks

 

[seco services]

More likely airlocked.
Try closing the hot rads leaving cold rads only and run should force airlock out.

 

[luvdiy]

I did as you say - turned off all rads except the 2 not heating. Ran the system and after a few hours the rads statred to work properly. I was unsure about whether the air release valves on the rads should be slightly open or not. I left them open assuming that any trapped air in the line would need to get out. Anyway all well now. Hopefully the system will be fine next time i turn it on. Thanks for your help.