We had a new boiler fitted in June and the system was flushed and balanced at that time. All working well but the rad in the master bedroom will now only get lukewarm. The first time it happened I assumed it was a stuck TRV so removed the head and tapped the pin. This seemed to solve the problem but it has now happened again and the pin on the TRV is not stuck. I have checked to ensure no air in the rad - Any suggestions please?