There are two valves, one at entry, one at exit. It's possible the exit valve is shut off, so even when the TRV is open there won't be any flow in to it because the cold water can't exit.
edit
turned out to be a load of air in what I didn't realise was the next radiator along.
All of our radiators are good, apart from the one in my study, which only heats up around the TRV although there's obviously hot water getting to it.
I've bled it, it's not the TRV and the radiators either side get super hot. The pin for the TRV seems to move up and down fine and not be stuck.
So rebalancing is the only thing I can think of, but it sounds like a right faff to be honest.
Any other ideas?