I had sort of the same problem....I found that the particular bullet I was loading was bottoming out on the tip of the bullet inside the seating die stem, instead of the ogive. The bullets had a sharper, smaller meplat that touched the bottom of the blind hole in the seater stem. It wasn't by much, but it was giving me a concentricity problem. I only found this after I started trimming meplats for uniformity and suddenly the concentricity problem disappeared.