Is the rad fed from above? If yes (and especially if there's any microbore tube involved) the drops may be airlocked.
Solution- close both rad valves. Open bleed valve fully, if you have modern radiators, remove the bleed valve completely, replace with a hose adaptor. Open lockshield fully, keep topping system pressure up while water is running into a bucket, keep running til at least 10 litres have come through. Close lockshield, open trv, repeat. With luck while doing this you'll get an episode of gurgling and air as the airlock clears.
Once you're done, close both valves, remove bodge, replace bleed valve, open both valves, rebleed rad, mop up spilled water. If rad now works, consider topping up inhibitor
Solution- close both rad valves. Open bleed valve fully, if you have modern radiators, remove the bleed valve completely, replace with a hose adaptor. Open lockshield fully, keep topping system pressure up while water is running into a bucket, keep running til at least 10 litres have come through. Close lockshield, open trv, repeat. With luck while doing this you'll get an episode of gurgling and air as the airlock clears.
Once you're done, close both valves, remove bodge, replace bleed valve, open both valves, rebleed rad, mop up spilled water. If rad now works, consider topping up inhibitor