Just replaced a single convector radiator with a double. Only the bottom half of the new radiator is getting any heat. There was almost no air in the rad when I went to bleed it. All other rads on the same floor of the house are working fine (as was the old one). All suggestions welcome.

Many thanks.

if its not too personal, what size pump do you have?

Check tank is full.
Bleed again.

Have you adjusted the lockshield valve? Its the valve you wouldn't normally adjust, if its closed or nearly closed it will prevent water from passing through the rad.

If you still have the old rad, count the turns until the lockshield valve closes and adjust the new to match. They allow the system to be balanced, rads nearer the boiler will have the locksheild valves closed more than the rads farthest from the boiler…stops all the water taking the easiest route through the first rad.

Not sure of pump size. I'll give the suggestions above a try.
Thanks for the help everyone.