In this order:

1. Change shells and see if something different will pattern different (you'd be suprised how different the size of shot, amount of powder, type of wad, ect can change a pattern).

2. Try a different choke tube (before buying a new "turkey choke" try shooting with your regular "modified" or "full" choke. If they pattern in the same area (with a slightly larger pattern) go on to #3. If they pattern well, you could use it, or then try a new turkey choke.

3. Get a new barrel or have a gunsmith straighten yours. Bent barrels are quite common.