You're having issues within 25-yards?
Bear in mind the cheap buckshot - Winchester Super-X and Remington is plain lead No. 3 buck. Federal makes a copper-plated No. 3 buck load that may help (Vital-Shok, Power-Shok is plain lead). I think one of the issues with 20-gauge is pellet deformation, the pellets are pretty light, the cheap stuff isn't buffered, and it goes out at fairly high velocity. I've seen a lot of shot patterns on paper that look like a reflection of flattening pellets resulting in bad spread at farther distances (it's why a choke is so critical in these guns).
The Federal loads are buffered between the shot (both power-shok and vital-shok), so you get less deformation. Vital-Shok is also a wee-bit slower at 1150-1175 and has the copper plated pellets. The Vital-Shok is the best patterning 20-gauge load I've shot, keeping things tight into about a 6-7" group out to 25-yards with no fliers (this is still running a fairly tight Mod choke). I like this load a lot - except for the fact that it's hard to find.
The Rio #1 Buck is a 9-pellet load, all plain lead pellets at 1200 fps. The heavier weight of the #1 pellets seems to prevent pellet deformation and through an IMOD choke it's close to the patterns of the Vital-Shok and it is more available and comes in 25-round boxes. So this is what I use for defensive purposes most of the time. It keeps everything inside the torso of a B27 out to 25-yards. That's good enough for my needs.
Of course - when in doubt, Federal TruBall slugs are available for 20s. I'd rather fling a 3/4 ounce slug than question my buckshot patterns.