The supply valve may be partially closed. Make sure the supply valve is fully open.
The supply valve may be corroded and broken and the valve seat not opening fully or at all. Check for proper operation.
Radiator is improperly sloped (too little). It should slop slightly toward the end of the radiator having the supply pipe/valve. Shim as necessary to achieve proper pitch of 1" for every 10 feet toward the supply pipe / supply valve.
Radiator is improperly sloped (too much). If the radiator is sloped too much it may cause sediment to flow into the supply valve and partially obstruct the valve opening. If that happened you need to remove and flush the radiator.
The air vent may be improperly installed. It must be installed vertically. Make sure it is not installed upside down, diagonally or sideways.