Problem with new radiator installed in adjacent room All aspects of plumbing questions and answers, help, tips and information

[Tezbedz]

I fitted a new radiator using tee joints of both feed and return pipes in the adjacent room.
The tee joints were placed in line on the existing pipes and new piping run through the wall to the new radiator.
The original radiator heats up but the new one doesn't, there doesn't appear to be any pressure at the feed end
What have I done wrong?

[Tezbedz]

Fitted a new radiator by taking t joints off existing flow and return pipes on radiator in adjacent room.
8mm pipe with soldered joints
Only a dribble of pressure at flow valve on new radiator, yet radiator in adjacent room is hot.
Could I have blocked a joint when soldering?
To investigate do I just unsolder a joint, there is a drain plug in the old radiator valve but I cant turn it using a bleed key.
Any thoughts really appreciated