I'm assuming the heating system was drained to install the towel rail. So it could just be air in the system, or an airlock. What type of boiler / system is it ?
The towel rail would still get warm, even if only half way up, if the heat was getting to the radiator.
Make sure both valves are open, and vent the radiator (with heating turned off) so that there's no air in the system. If this doesn't work, turn on heating and close all wheelhead valves on other radiators, this will hopefully clear any airlocks.
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