Rads upstairs work ok, downstairs they dont, hot water is OK

[MartinCoutts]

Hi,
Please can you help, the whole central heating system was working OK. Now the upstairs works OK, but downstairs none of the rads are getting hot (or even slightly warm). I have checked the pump which is working, the two diverter valves are working. I have tried setting the diverter valves in postion A, B or mid position and in several combinations. Any suggestions as to what the problem may be?
Thanks Martin

 

[Nobby^1]

are you sure the pump is working and is it working to full capacity?

 

[breezer]

more info see here

 

[MartinCoutts]

THE PUMP IS WORKING 100%. IF I CHANGE THE FLOW RATE I CAN HEAR IT ALTER THE THROUGHPUT.