You need to pray to RNGesus more often I guess. Newly planted ones will randomly have Resin Holes just like Wild ones will randomly have Resin Holes.
They have a Sapling Chance that is so low that it averages on 1 sapling drop per Tree, ofcourse averaging means 0 and 2 can happen just like 5 could happen.
Also you are NOT allowed to shear leaves off the Trees, because then the Resin Holes will stop working entirely, even if you plaster the Leaves back onto the Tree.
So it's just bad luck so far? OK. I only sheared the wild one I found that didn't already have a resin hole, and I chopped down another one or two that didn't and got no sapplings.