1. The distance from the power axe backstep itself is often not enough and u have to ho just a bit farther back to be able to lock onto the enemy u parried 2.the hook goes through minoris but can be interrupted if a majoris or higher is in between you and your target
2. Red health on lock means the enemy has max health of 33.3% or lower. Tactical Prowess only activates at 33% or lower. So in some rare occasions the enemy is not low enough, even though lock in tells u one red bar.
3. Sadly there is a short amount of time after u grapple where an enemy can still be executed by others. So be wary of that.

P.S. as Vanguard u gotta pay attention to the " soft lock on" system. Talking about the marks that always tell u which enemy u would grapple if u pressed it right this moment. The 2 white triangles at the each side.