Bathroom towel rad not heating fully?

Potterton Netaheat Boiler / Towel Radiator.

One of the pipes connected to the radiator heats up, but the other remains cold and radiator is cold most of the time. If the heating is left on over night, it heats up more, but not fully. Have tried bleeding but does not help. There doesnt seem to be air in the system either. Any suggestions?