Flare, bell, bullet diameter, bullet nose profile, seating depth, final case diameter. Your problem is one or more of these things. Crud buildup can be a cause. Case buckling could be another.
This is one place where a bullet nose comparator can help find or rule out the cause. I use the Sinclair lug nut style, cheap and effective. You can make a comparator with a drill bit size that matches the land diameter, and any chunk of flat sided metal.