If the top corners of the rad get hot but not the bottom middle then it’s rad sludge as it collects in an upside down ‘V’ shape. Are both radiators near to each other? I’d say flush the rads with a hose pipe after you have tried this:
First thing to try is switching the programmer to heating only, turn the stat right up and shut every single rad off bar one of the ones you’re having trouble with.
This should get the rad hot as the pumped heating water has got no place else to go. Assuming this does get the radiator going open the other problematic rad before shutting the one that just got hot (save burning out the pump).
Trapped air in a pipe feeding these radiators is most likely, it catches people out as they bleed their rads and get water, so assume they are air free.
If you have already tried this or it doesn’t work the next thing to look at is the pump.
It’s most likely a red grundfos ups 15/50 (same but blue if it was a BG part). It will have a flat head slot on the front of it the size of a pound coin, in screw it all the way, this is how you bleed it. Water will dribble out at a slow and steady rate so have a bowl handy, grab a screwdriver until it’s scraping the impeller and try and stop it from spinning. If it’s easy to stop then the pump is on its way out and it’s a circulation issue.
It’s at this point I’d be thinking it’s a blockage, if both radiators are next to each other it’s going to be sludged pipe work. There are options such as power flushing, chemical cleaner or re piping those rads from the carcass pipework where it’s known to be working.
Make sure your pump speed is up to 3/3 as well. Do you have a gravity system or pressurised?
