Why are only 2 of the 14 posts in my radiator heating up?
I am assuming it is a steam system? Verify that the radiator is installed with a slight pitch, the inlet pipe side should be slightly lower than the exit. If there is a vent installed you should hear a hiss as the steam forces the air out and lets the steam in. The hiss should stop when the steam reaches the vent.  If you can remove the air vent and shake it, you should hear a rattle that is normal. You can clean them by soaking in CLR to remove the scale or just replace it. They cost about $15 at the big box store.