Why does the radiator in your lounge not warm up even though you have bled the radiator?

I'm supposing all other rads on the system are hot? If so, it sounds like you either have a flow problem, check wheel-head and locksheild valves are open. If they are you'll need to determine if water is actually passing the valve when open by un-doing the joint. I assume when you bled the valve air followed by water under pressure followed? If this occurred it's likely there is suitable flow and that the radiator itself is blocked with sediment.

The most cost effective measure is to remove rad and flush out individually with hose pipe, remembering to lock of valves before hand.

Observe how much silt/rust deposits comes out. Flush till clean and refit, open valves and re-bleed, see how you go. Are you using inhibitor in the system do you have an engineer service it every 12 months.