--one's B12 alone, completely separate from any other supplements, and at least two hours after food or one hour before.

The cobalamin molecule is the largest molecule our body uses as a nutrient on a regular basis; it is VERY easy to interfere with its absorption, especially if one has problems in that area to begin with. Even taking it with other B vitamins tends to set up a competition that the larger B12 molecule is likely to come out on the short end of.

I would try taking your B12, in whatever form you're taking it, completely separate and alone for a number of months and then compare.