One thing with progressives, you have to be consistent with the vibrations thru the cycling process. Same with any powder thrower, has to be a consistent vibration cycle. You can tap the bar or the jug, but, if you do, do it every time, the same way. The way you bottom out the lever, has to be the same every time, keep the shot hopper within a close level to where you started, don't run it out. And like mentioned already, static is an enemy as well.
Right now, you may be averaging 14.8, according to your info, check the average over about 50 rds, see where you are, as to where you want to be, you may have to adjust a bit. You can hone out smaller bushings to fine tune it for that load. All you can do, is to be consistent, and try to cut down the spread as much as you can, it will always be there though. Shotshells are pretty forgiving that way.