2 Radiators Not Heating 2 Pipe

mugen

Have a very strange problem at a customer's house. Numerous companies have been there now its my turn to be schooled. Here's the problem.

2 radiators one on second floor and one on third don't get hot. All other radiators get hot. 2 pipe Hoffmann DL system. The last companies replaced all traps. 

When I arrived I took radiator out I could hear gurgling in pipe. Blew down pipe with nitrogen after a little while steam cane out and both radiators got hot. The customer called and said when heat shuts off it didn't get hot again. Went back today removed radiator gurgling again. Removed cross over trap from main and return line  blew line from radiator nitrogen came out of third floor radiator and out of main where I removed trap. It makes no sense. 
The pipe that is supplying steam to the riser that goes to the problem radiators is not insulated and in garage and pitched slightly away from main and riser is last on pipe. I feel this might be the cause. To much condensate in this pipe causing issue with steam. Would this be plausible. 

Any suggestions would be appreciated. 

Steamhead

There is a low point somewhere in that piping that is trapping water and blocking the steam. Find it and re-pitch it.

Jamie Hall

Is there a drip on the end of that pipe which is sloped away? If there isn't, there needs to be one... either that or a water seal loop to the dry return at that location. Assuming the dry return exists... and slopes back to the boiler... despite the best efforts of some engineers, water only flows down hill.