Hi Angry Bear
Case length should have no influence on COAL because it is determined by the distance from the seating punch contact point and the shell holder surface when the press ram is in the fully closed position. However, if the seating punch either deforms the bullet tip or somewhat deforms the bullet ogive (may be very difficult to see) then you will likely get differing COAL which is then related to seating resistance. This may of course be inter related with neck length, neck ID, brass hardness etc. I have had exactly the same problem as you with a number of calibres and bullet ogive forms. The last one I had was with my standard Hornet. I just made a nose punch to suit the bullet I use and problem instantly solved. Unfortunately I don’t know if you are able to do the same with what equipment you have access to.
Cheers
pjinoz