Technical

Can anyone please give me advice on this problem. I have an A4 Mallard little used that runs perfectly. Its regulator works ok and with the body shell removed the regulator control arm can be seen to move as directed by the controller. When the regulator is closed the red light comes on and as the regulator opens the red changes to green. However after about 4 clicks the green light goes off and remains off until the regulator is bought to a close again. The loco performs normally in every other respect. I have tried to identify a loose contact without success. Has anyone else had this problem or an idea were the fault lies? Much appreciated.