Sounds to me like more of a case of of an individual animal and a bullet placed in a spot that does not deliver a DRT result (i.e. brain or central nervous system = spine at front shoulders and forward), than a case of a heavier bullet being needed to provide the DRT result you were expecting. Just my 2 cents...
No two animals will react to being shot in the exact same way, as like humans, they are all individuals with different personality characterisitcs.
For example, most of the caribou I have harvested have been quick, cleean harvests with one shot. And they are not known as being overly tough to harvest, like elk or nilgai.
But my first caribou took five rounds into the lung/heart cavity from within 100 yards before sucuumbing to its wounds. All five shots should have done the job individually, but that caribou was tenacious and not about to give up the ghost!
Note: I am not one who will wait for an animal to expire for the sake of a one shot kill. If they are not expiring in an expedient fashion, I will shoot again, as I was taught to not let an animal suffer.
For many years now, if I need a DRT result that needs to quickly anchor an animal so that it cannot make it into a bad position, I will use a high shoulder shot instead of a double lung hit. This had worked very well for me on moose, elk, deer, mountain goat and sheep. It takes some observation of the animal's surroundings and forethought before shooting.