Third floor radiators wont answer

toddj_5

I have a friend who has been having a problem with the third floor apt steam radiators. Building is a 3 unit with a steam system having one zone for all units. Thermostat is on the 1st floor. Since heating season began the radiators on the third floor havent heated unless my friend drains the furnace via a return line (b/c the furnace valve won't drain) and then raises the temp to 70 and the heat seems to work for a day or two..then the radiators stop working on the third flood again.
Also FYI, they replaced the vents on the third floor. The pipes in the basement are insulated. They don't have an auto fill on it.

Any ideas what the problem could be? How to fix the problem?

Thanks you guys seem to come up with the right answers.