First: What choke are you using?
Second: How far away are the ducks when you shoot?
Third: Pattern your shotgun and see whats going on.

I'm a pretty good shot my self, but when it comes to duck hunting I found out with the cheaper shells I cripple more birds than with a better shell. In the long run it ends up being more expensive. Personally I like to shoot Winchester Drylocks #2 or Remington Nitros. I do not like the Winchester Xperts. I know what works for me. I would rather spend $2-$3 more for a box of shells and knock them down with one shot than have to take three to kill a bird with a cheaper shell. Just my opinion.