heating?

hi i am having some problems with our central heating...
to be brief we are in a three storey townhouse and although the boiler is old it still works well.Its a potterton flamingo 40. The problem is the two radiators in the top bedrooms seem to be getting hot but there is no heat reaching the two mid floor rads or the three groundfloor rads. The circulating pump became quite noisy so i replaced it with a new one together with the valves this did not cure the problem so i have drained the system and added 1 litre of sentinel 400 sludge remover re filled and bled the system. (Still not sorted) I have managed to get a luke warm feel to the mid floor rads but thats it. The diverter valve appears to be working as the pipe from the bottom of the valve gets very hot when it switches to the central heating although i am unsure whether the valve is opening fully is this a possibility? The diverter is a drayton MA1 any help or suggestions would be gratefully accepted...