I looked at your profile. Certainly not going to tell you you’re doing anything wrong, just that the issue you’re experiencing has a lot to do with the fact that you’re playing an Impale build.
Impale is a high single target damage build, and these sorts of builds have a much steeper “damage wall” than other builds that can deal more aoe and leverage AD + mob density better.
In other words, with Impale, you tear through GR levels quite easily up to a point, and then it pretty suddenly becomes hard, and then impossible, within the span of only a few tiers.
It’s the same with all single-target-focused builds, for instance the new H90 Frenzy Barb.
On the plus side, adding mainstat via augments and paragon will make those “impossible” clears just “hard” and those “hard” clears “easy” more directly than if you were playing an aoe build.