Wet Return Pipe Occlusion
The wet return piping on a steam system, (be it one pipe or two pipe steam) is one of the lowest points of the steam system, and is always under water. The pipes start to close up with rust that is created in the wet return piping itself, and also deposited in it from the condensate returning from the piping upstairs carrying rust down to the lowest points in the system as the condensate makes its way back to the boiler.
Over time the pipes start to occlude in a similar fashion to the arteries of a persons heart. Eventually the occlusion is severe enough for the condensate to have trouble returning to the boiler. When this happens the boiler may experience issues of flooding, as while the condensate struggles to get back, the automatic water feeder has already replenished the missing water. When the system shuts down, all the water will eventually return to the boiler and the result will be a flooded boiler. This may or may not induce a 'no heat call'.
A flushing station can be added to wet returns to flush out crud under pressure, and help keep the returns open. But as you can see in the pictures, once it gets bad enough, flushing is not going to remove it. Pipe replacement at that point is the only realistic option.