Might - just might be that all the heat has been used up before it reaches this cool one. In that case the solution may be to balance the rads... adjust the lock shield valves to obtain a 10-11 degree C drop between in and out. One way is to do it without measuring is to fully open the control valve and fully close the lock valve - then slowly open the lock valve to give a reasonable temperature. Then on to next rad... Think your're supposed to begin with first radiator - the one closest to the circulating pump.
You might try turning off all rads except your cool one to see what happens.
Peccavi
December 2008