It's not the bullet, I've killed 35+ deer with SST/Shockwave 200/250/300 grain bullets. This is in .45/.50 cal rifles, .50 cal pistol, and 20 ga 250 gr SST Sabot Slugs. None made it over 60 yards, all pass throughs except one, with exit evidence that all expanded. The one bullet I recovered was a 200 gr Shockwave with Black Mag'3, shot through both front shoulders at 115 yards on a mature whitetail, and was found under the hide on the offside shoulder. Bullet expanded over 2x caliber and stayed together after going through all that bone.
Old Pellets and Low Drag (EZ Glide) Sabots are the problem. Pellet boxes are not sealed from the factory, and start degrading from day-one. You might have purchased them a year ago, but they could have been made a year or two before that? We read these stories every year of the bullets that don't expand, yet ironically the deer all died and were recovered. Then you have the guys trying to use one 50 grain pellet and the Low Drag sabots for an easy loading and light recoiling load for their kids. This is a recipe for poor performance, even though it looks good on paper. Don't fall into that trap!
I do not use pellets or Low Drag/Easy Glide type sabots for hunting. The so called failures that we read about every year are using one or the other, if not both. As mentioned earlier, even at poor velocity that doesn't promote full expansion for easily followed blood trails, the deer were recovered in relatively short order. Shoot those bullets through a chronograph with the same sabots, and I think you'll find out where the root cause of the problem exists.
Just for the record, I've never had a SST or Shockwave "pencil" through or "grenade". I think I have enough examples and experience with these bullets to have developed an informed opinion on how they have worked for me. All shots were in the heart and lung area. I've used between 80 and 120 grains by VOLUME of BlackMag'3/Triple Se7en/Blackhorn 209, or the factory 20ga SST. Never pellets, and never loose fitting Low-Drag/EZ-Glide sabots.
You're on the right track going to loose powder, and those bullets will work just fine with the right load and sabot. So will the XTP's.