you didn't mention how low it was shooting.if you getting a full bead with it aiming just below the head. then try covering the head with the bead and see what happens.
I also have a CVA double in 12 ga. that works great.

you just have to try different points of aim to get it right.I did put some fiber optic sights on mine and love them. they have two green dots at the back and a redbead at the front, really helps on the low light mornings.
you may try that. I took my dermial tool and ground down the hollow in between the two back beads so I can get the front bead to fit tight on the head.

hope some of this helps.good luck,it's fun to shoot and not know if he is down till the smoke clears.