fitted new radiator - not working?

We fitted a new radiator and new thermostat in the living room, the rad is full of water - no leaks, but when all the other rads are on, this rad stays cool and even the pipes to it are cool? Don't understand it. We have bled all rads. We checked the thermostat bit in the middle goes up and down. Is there any thing else we can do?

R Slater
May 2011