As above, just because the pin moves doesn't mean the valve seating isnt stuck, if tapping it with a hammer doesn't free it up it will need replacing.
To test everything that has been stated.
1: keep the trv head on but turn it down to 0
2: wind the lock shield valve right down shut.
3: remove the rad, now you can also check whether it is full of sludge, the water coming out will reveal all
4: hold a tub/bucket near the trv and turn it up to max, you should get a good flow of water coming out, be careful.
5: if no water comes out replace the valve (this will require you either releasing the pressure if a sealed system, air locking/bunging if it's open vent, or draining if you don't know how to put an air lock on)
6: if you get this far and all is ok at the trv end try the same with the lock shield valve, highly unlikely this has failed, but I replaced one last week because the cust had broken the spindle from the seating.
7: if all is good then you have an issue with the pipe work, being plastic you can expect it at some point in the systems lifetime.