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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Dave, Are your heating loops regulated by zone valves? The valve to the lower level might be stuck. Or, you might have a circulating pump for each loop instead of zone valves. In that case, make sure the pumps are running when the t-stat calls for heat. Bert

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