Give it a trip or 2 more, and if you continue to shoot a little lower, put the #3 sight on. I didn't have to do that with either of my Walthers, but I did have to do it w/ my SW99. It also kinda depends on the ammo U use (weight and brand). I typically use either WWB or CCI Blazer 115gr.
Also, I "cover" the center of the target w/ the front sight dot. If U use the top of the target and put it at the bottom of the center, that could explain it.