A 2 mildot drop at 55m from a 30m zero sounds about right - the left-right thing doesn't sound right.
Some questions for your your zeroing problems...
- What scope have you got on there?
- Are you setting the parallax for each range?
- Have you tried swapping the mounts round?
- Are you able to shoot in the opposite direction (to check it's not the wind causing the left-right shift)