Radiator only warm no hot

Check the T.R.V. & locksheild on the radiator is working & if it is , it is worth balancing all the radiators with a dual temperature measurement device , to accurately adjust the locksheilds.

You have a bit of an air lock in the return pipe from the rad. As an engineer we would normally try pulling the air out via a drain off valve if there's one on the rad or cracking a nut on the return pipe to dispers any air catching any water into a bowl. What you can try and is often effective which is also another way that we try is to shut off some of the other rads completely not by the trv side but by the locksheild valve. Stopping the flow of water/heat to other rads forces it towards the cooler rad, this is often enough to get full flow of water / heat into that rad. Once your happy its hot, slowly open the closed locksheild valves 1 turn. All rads should be warm. Of course if unsuccessful you will have to get an engineer to pull the air out and balance system.