1 radiator out of 6 did not heat up fully?

having done the following i.e bleeding repeatedly from both sides, balancing them repeatedly, changing both flow & return valves and replacing it with new radiator, closing all other radiators (both flow & Return Valves ) except the defective one and then putting CH fully on to highest temprature setting, it still does not heat up.
one mystery is that both flow and return pipes on this radiator get untouchably hot, but the hot water does not go through to the radiator, please help