Central heating working downstairs but not upstairs

[Castleirwell]

Hello all, wondering if anyone can enlighten me with the above issue and any possible fixes are much appreciated! I have an oldish system i guess, boiler and expansion tank in loft. The rads downstairs are all boling hot, but none of the rads upstairs are gettting hot. We've bled them all, to no avail.
Now could this be a broken faulty pump? I only had it replaced 3 yrs ago. Or could it be that there's not enough water in the system? or something to that effect?
Any help/suggestions much appreciated.
Thanks!

 

[kevplumb]

shut the ones that are working off first

check the header tank as well

 

[twgas]

possibly a stuck anti-gravity valve, especially if its gravity hot water.