A couple of questions before I get drawn into this one (after all it's what I do for the day job
1. What make of radiator valves do you have installed?
2. Is the pump on the flow or return pipe to the boiler?
3. Has the system been extended at any time?
4. Do you always vent the offending radiator with the system turned off and how much air do you get out?
5. Do you have a fully-pumped system or gravity hot water and pumped central heating?
6. What colour is the water in the small heating feed tank?
7. How old is the system?
If you can answer the above before
you start "adjusting" then I may be able to shed some light on your problem.