Danfoss Ras-D2 Valve Problem?

I have just had a brand new heating system fitted in my house; everything is working as it should. The problem I'm having is one of my TRV's are not allowing enough water through, I have replaced the sensor head with the factory dust cap, which is still useable, and found that the rad is working correctly, nice and hot as it should, and then reversing the process to which the rad returns cool. I have swapped the sensor head with another from an alternative rad, so it seems to point me in the direction of the vavle body. These valves are brand new and have been fitted by a registered heating engineer, could you please help on what to do or check for next, Thanks for your time.....

Edward
December 2009