First thing I would do is check your choke and clean it...
Especially with tight chokes and thick plastic wads, you can get some plastic residue on the choke that can effect patterning (although I have never seen it affect patterning so drastically).
Also, if there is grime in the choke, it is possible it is not seated all the way, and cleaning the choke and threads, and barrel might make a difference.
Also, are you taking the pattern density at the center of the pattern, or are you taking it where you are "aiming the gun?" If for some reason your sights or gun is a bit off, you obviously will not get the pattern density where you are aiming the gun.