We often see air vents installed incorrectly on a steam radiator. These radiators can be installed for both steam and hydronic systems and therefore have two available ports. The top one is for a hydronic system bleeder and the bottom one is for a steam system. However, many people don't know this and use the top port for the air vent installation.
This blocks half the radiator from warming up because steam is lighter than air, it travels right up to the air vent and closes it leaving half the radiator without steam.
Below is the heat distribution on a steam radiator with the air vent incorrectly installed on top of the radiator (upper left here).