Have you Cleaned the chamber area? Even a small round count could foul that area and f-up accuracy. And did you adjust the torque as I suggested on RFC? Around 15-20 inch-pounds should be enough to hold without stressing the action, too much can warp it OR the stock. And if you get the 'new floor plate' don't crank down on that - the torque s/b kept low , as I said. Why even replace it ?? And have you tried to ask someone to 'watch you' and see if you're unintentionally doing something 'wrong'? "Jerking" trigger, flinching, not holding 'long enough' ?? Lotsa possible things.
Finally, it may be that you need to try more brands, some barrels are picky as Hardluk1 says.