Drifting

Do you see a covering that arm that is dropping? You've got vertical drift, there should be a cover you can remove, maybe half way up the arm. Pull off that cover and you should see a castle nut, if you don't, move the arm up until it comes into view. If you don't have a spanner wrench, you can use an Allen key or screwdriver to adjust it, just put something between it and the hole so you dont scuff it.