I get very good accuracy from Fed 3" mag 3/4 oz 1450fps barnes expanders, clover leafs at 50 yards when I do my part.
One thing that came to mind, you said that these grouped well last year. Are these the same slugs that you took hunting last year? Did they ride around in a truck or in your vest? Did they go in and out of the house? If so they could of drawn moisture cooling down and heating up over and over again. If so I'd get a new box and try them.
I buy 2 boxes, 10 rounds, a years. I put 5 rounds, one box, in the loops in my vest and the other in a zip lock bag in my vest. The following fall I shoot the 5 "loose" rounds to foul the barrel (get any oil out) and then use the "zip lock" 5 rounds to verify my scope's setting.
Technically I should buy 3 boxes a year, one to have in my vest loose, one in a ziplock, and one to test the scope so the box I test my scope on is the same "lot number" as the ones I'm shooting...
Anyway, slugs can draw moisture pretty easy, if you're shooting old slugs that might be your problem.
What was the old saying, "shoot straight and keep your powder dry."
later,
scruffy