we have had hard dogs bring cubs to the enterance just like a vixen would carry them, not a mark on them, and not just one, sometimes the whole litter, and not just on one occasion either... like wise iv seen lurchers refuse red cubs that have bolted, yet they do rabbits and adult foxs no prob..... nothing as strange as nature
personally i would have tryed it again, if it was not letting you down the rest of the year