Steam radiator problem

I hope I'm in the right board here. I'm having a problem with one of my radiators it doesn't heat up, well only about to the third rung. I have an oil fired furnace/steam heat. This radiator is on the 2nd floor, it's a bungalow house built in 1926 with one room upstairs. All the other radiators downstairs are working fine. The furnace had it's regular Fall maintence and I clean the clear tube once a week and add fresh water. I went and bought a new valve for this radiator but it still won't heat up all the way. It's cold in that room all the time.Do I need to call a plumber or is there something I can do to fix this.
Thank You Dave
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