Don't know if you're still trying to pattern your shotgun, but I used to do a lot of Trap & Sporting Clay leagues so I happen to know a little about this.
Are you using factory ammo or reloads? At what distance are you recording your pattern? Have you tried different distances?
If you're using factory ammo there's really only 3 things you can do;
1. Like Dale said, try using a different brand
2. Keep trying different chokes. I prefer Improved-modified.
3. The expensive option ... try different barrell lengths or get a buddy that has a longer or shorter barrell and see what happens.
If you're reloading, well, there's a whole lot of issues that can come up.