There are a number of reasons why a single radiator will not heat up compared to the rest, below are some reasons:

Assuming the rest of the system works, i.e. all other radiators heat up,

here is a non exhaustive list of suggestions:

1. System badly unbalanced.

2. Knackered radiator valve doesn’t really open.

3. Pipework clogged.

4. Pipework capped off and not connected to the system.

5. Pipework spur has been closed off by isolating valves elsewhere.

6. Radiator is on separate zone that isn’t powered.

7. Radiator seriously bunged up.

8. Radiator not bled.

9. Radiator above or close to level of feed/expansion tank having been installed by brain dead bozos.

Did this radiator ever work?