Steam Boiler radiator problem

Had a new Weil-McClain steam boiler installed in my 1928 house. It's been a huge struggle. Voilent banging pipes and water gushing all over floor. Plumber tried many things and finally repiped correctly (according to manual) and it's now working fine except...one radiator comes on only if I turn the thermostat up 3 or more degrees at once. Even then it's the last one to come on and sometimes only warms 3 or 4 columns. But sometimes it heats completely. When maintaining the same temp., it never works at all. Of course most of the time I keep it at one temp so this means most of the time it's cold! There's no problem with the radiator itself- plumber confirmed today that the radiator and vent are fine.
The radiator is located in an addition on the first floor, surrounded by windows and is my favorite room. Today plumber turned vents to 1/2 positions on other radiators on first floor thinking the thermostat reads warmth from them before this rad has a chance to come on. Interesting theory but, once again, when I kept temp set and a new cycle came on, the rad didn't come on. I've been on the phone with Weil-Mclain technical services several times and they can't figure this out, either.