I am not a tech by any means but when I got my dirty harry pinball from a guy recently, the same thing was happening to the left flipper. I could only scroll with the right flipper and not the left. So I looked at the flipper optical board? and cleaned the small bulbs with a soft tooth brush. The pin had been sitting for years with no maintenance. That seemed to do the trick since it worked perfectly after that. I was fearing the worst but it was just dirty. Hope your issue is that easy...