Cold at the top
Trapped air. Bleed the radiator with the heating off and the system cool, then top the boiler pressure back up to around 1.2 bar. If it keeps needing bleeding, air is being drawn in somewhere and the system needs looking at.
Cold at the bottom
Magnetite sludge — iron oxide settling out of the water and blocking the lower waterways. Bleeding will not help. This needs a power flush or, on badly affected systems, radiator removal and cleaning.
Some radiators hot, others cold
Usually a balancing problem. The lockshield valves need adjusting so flow is shared evenly rather than short-circuiting through the nearest radiators to the boiler.
Whole system lukewarm
Check the pump, the diverter valve and the boiler flow temperature. On older systems it is often a failing pump running slower than it should.
Prevention
A magnetic filter fitted on the return, cleaned annually, keeps sludge out of the boiler and is one of the cheapest upgrades available.