Radiator cold at bottom

Hi, we had an new boiler/radiators installed in Sept (system was flushed). The double radiator in the lounge is boiling at top but only lukewarm from the middle downwards. The pipe coming into the left of the radiator is boiling but coming out of the right is only warm.
I can remember the first time I bled this radiator lots of white foamy stuff came out with the air. Don't know much about radiators/valves etc so any info would be appreciated

common problem

sludge or needs balancing

Or if you have a thermostatic valve it may have jammed near shut during summer

look at the five similar topics at the bottom of the page before they go

Was the system balanced after adding new rads?

if the return pipe is a lot colder, it is a matter of balancing