I purchased the upgrades that enable the hanging kills (stringing guys up from grappling hook points) and the being able to kill guards while hanging underneath them from grates but it's not prompting me to pull off the moves. I purchased the moves, then had to restart a checkpoint, but when I went back to buy them again, they were already registered as purchased. I'm hanging from a grate in the floor underneath a guard and it's not giving me the prompt to kill them. Is there a place in the menus I can see my unlocked skills or can I only look at them in the unlock screen?

are you pressing the thumbstick in the direction of the guard? if you just stand there it doesn't do anything but if you press toward him it should appear the prompt for triggering the kill