I do not own a Sota,but have worked on a lot of machinery that uses belt drives.Im guessing that by putting a washer you are changing the level between the pulley in relation to the platter.If all points that contact the belt are not correctly lined up to each other the belt will want to pull one way or another,tension could be a factor too.Can you vary the position of the washer,or use a thicker washer,is there a way to adjust the level other than using a washer,to find the spot where the belt rides where it should?Maybe someone who has more knowledge than I on this table will chime in.Good luck,Ray