Downstairs Radiators not heating

Hi,
I have a gas boiler system. Recently, we were without heat for about a week, while we were getting a new solenoid fitted to the filter on the boiler. Since this was fixed however, when the heating is on, no heat is getting to the downstairs radiators, while the upstairs radiators are hot.
When I turn off all the upstairs radiators, the downstairs radiators heat up no problem, but will not heat when both upstairs and downstairs are on simultanously.
I tried to bleed the downstairs radiators in case there were airpockets, but water came out immediately, with no hissing air.
Any ideas? Before the heating was off for a week while the filter was being fixed there were no problems.
Thanks in advance, David
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Radiators may/prabably need balancing Turn all the upstairs rads of by the lovksheild valve. Then turn each one on by a quarter. Hopefully all rads will now be on. If not it is a case of adjusting upsatirs till all rads are working
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It is very unlikely to be a balancing issue, as they worked before and the balancing adjustment wasn't touched. Setting all radiators to 1/4 on is far more likely to bugger up the balancing than cure it.
Christian.
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Is the pump working - convection could heat the upstairs ones but not the downstairs.

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