I use to have the same luck but try getting you a Poland china boar and mixing in your blood it will increase your litter size
I try to raise woods savvy hogs. I think the domestic mixed in would go pretty quick in the wild.
I raised a bunch of mule footed hogs and turned them out to never see again!
It is working well cause I caught a sow yesterday I turned out 2 years ago about doubled in size and had some pretty blue and colored up piglets.