Glyph of Resilient Grip

Anyone else having situations where it doesnt do its job?
For example, Tank in Tol Barad. I was just running around farming herbs when I accidentally pulled him so I ran up on top of that rock in front of where Captain P. Harris is on the sunken ship. While up there I still had aggro but obviously he cant leave the water. So I tried death gripping him up and it said Immune. Then I remembered how if you're on the rock, he will be immune to all attacks after the initial attack. So I figured he might be immune due to that. But when I went in the water and tried to death grip him, it still said immune and the cd did not reset. I tried it multiple times on tank and it never resets because hes immune.
had similar issues with that, but i believe death grip also makes the target focus on just you for 2-3 seconds. I think the Immune comes from that part.

i could be horribly wrong tho.
It does cause high threat. In the spell tooltip it says "forcing the enemy to attack the death knight for 3sec." which is where the threat comes from. I just dont like how the glyph says it will reset the cd if the target is immune and it actually doesnt reset it on certain mobs. It should be all mobs.