Assuming your are using a pattern board of some type:
try placing a folded up cloth on the top of the comb/stock.
perhaps you are bringing up the comb to far with the factory stock. Are you tall ?
does the shottie have a rib on the barrel and a mid bead. if so, with mid bead, how are you lining up your two beads..? muzzle bead really should be slighty higher, forming a figure 8 with the two beads.