My house is pre-1900 and large. I have a steam gas boiler. I have a problem with a two radiators that don't heat when any vent is attached. I have changed vents, checked pitch, etc. Quite by accident I discovered that if I remove the vent the radiator works beautifully. However, as soon as I attach a vent, any vent, the radiator does not heat.
I have changed vents, vent sizes, vent types, etc.