If the backing went on level, something must have changed when you spooled the main line. What type knot did you use to attach the main line? See if the knot dug in or created a lump on the spool. Did you run the line through your fingers putting tension on the line? If so, you may have been unknowingly pulling against the level wind. If the level wind is suspect, apply light finger pressure against the line guide in the opposite direction it is moving as you reel. Feel for rough spots or hesitation, especially at the ends when the guide changes directions.