all radiators heat up then one goes cold

[crossmister1987]

when the central heating is turned on all the radiators are hot, (bottom to top)
but after the room reaches temp and the boiler goes off (just the pump running) when it drops temp in the room the boiler heats up again all the rads heat fully other then one, even tho this was hot on the first run, ive checked the boiler pressure, bled all the radiators multiple times only water comes out. ive tried balancing the system to no luck as it is still doing this ive also tried to turn the pump up to setting 3 but is still doing the same any once know what is causing this and how i could possibly fix it, thanks

 

[ianmcd]

does this radiator have a Thermostatic radiator valve fitted ? if it does then remove the sensor head and see if it works ok with the the sensor removed if it does then you have a sticking valve