What can i do with my cold rads?

[cgurney]

I know common problem but i don't know what else to try.
My house has 10 rads on a combi boiler - the boiler seems to be working fine but not all the rads are hot around the house - of the 6 downstairs ( on the same level as the boiler) 2 are cold even after being bled and of the 4 upstairs 2 are cold and the other 2 are cold at the top - these probably need bleeding but they are single rads with no valve to bleed.
Nothing new has happened to the heating system in the past year to warrent any abnormal behaviour and I just don't know what to do please help!

 

[gigz]

The two rads upstairs will have bleed nipples but they may be at the top at the back (feel the topside facing the wall) you will need to bleed downstairs first then upstairs with the boiler switched off. Top the pressure up to 1 - 1.5 bar then turn on heating. As the rads start to warm turn the hot ones off to force hot water to the other ones. You will probably need to flush and add inhibitor as well.