I used to have terrible check slap with any modern shotgun I used. The typical stock dimensions on newer guns just didn't fit my face shape. It wasn't until I started shooting older SxSs with much more drop at the heal and the comb that the problem went away. I now shoot guns that have 2 3/4" or more DAH with 2 1/8" or slightly more at the comb where my check touches. It's not uncommon for me to take a file to the stock of a "new" gun and drop the comb down and then refinish the entire stock and forend. On guns with modern dimensions I always saw lots of rib and had to float targets. Upon firing the force lifts the gun into your cheek more than it should if the comb is too high. And because I saw so much rib, I forced my cheek down hard on the stock to get as low as possible and this just made it worse.
