I have a one pipe system and just had a new Vertex vertical radiator fitted. It's the last radiator in the circuit. The radiator I had replaced for the vertical radiator worked like a dream for the past 11 years. The new radiator heats up when the system is first switched on and then the bottom half goes cold. The return pipe to the boiler also goes cold. It's like the flow stops. All the other radiators get hot. The plumber came back to bleed the system and flushed it clean. As soon as the system is switched back on the radiator heats up very nicely but then cools again. All the other radiators have been switched off and switched on again and pump has been checked. The plumber has no idea why this is happening and he has 40 years experience. Does anyone have a clue why this is happening? A wild guess on my part is that there is some sort of blockage in the return pipe.
One pipe steam pipes have to maintain a consistent pitch down from the radiator to the boiler. My guess is when you put the new radiator in you let that existing pipe drop down a little bit. Now it starts to get hot but once the little well of condensed steam(water) forms at the low point, it gets blocked and stops getting hot.
Another possibility is that the vent is closing too soon and not letting all the air out. Should be an adjustment.