problem is the spring is not centered , when you tighten up the shaft nut the whole thing is moveing, tighten it up full then release it a bit,1/4 turn , use a drift or something to tap the spring central then nip the shaft up again and check alighnment, if its right both arms will move the same amount,
, you used to be able to get a tool to hold the springs central while tightning, dont know if you can still get em maybe LBS can help you out