I presume your poa is the center of the cardboard. If so, your hits aren't that bad. I've shot some brands of slugs through the Vmax that wouldn't have been on the board at 50. But if you're using a front rest for the gun you should scrap that and hold the gun like you'll be shooting it for real, but support your elbow on the bench. You will likely come up with a much different poi. And you really need to try some different brands, weights of slugs in your gun. Just like a particular load/bullet can be in perfect tune with your rifle barrel, the same will happen with slugs. Case in point, I can shoot anything in my Vmax out to 40 yards and get a hit; Fiocchi low recoil 7/8 and 1oz out to 75 yards no problem; but if I want to get a hit at 100 yards I better have the Fiocchi 1oz low recoil in the gun.
Once you figure out the slugs that match your gun, then go practice shooting targets offhand. It's a little different than from the bench.