Sometimes CS/Anni does not cast even though I am facing the target, in melee range, with enough fury. It doesn’t show any errors, and shows visual feedback in the bars that I am pressing the button. But my character just doesn’t use cs/anni and keeps doing melee swings. It takes several seconds of strafing my character left and right or wiggling my camera (I actually don’t know if these help) until the game lets me use the ability again.
You also want to add, when casting Fel Rush, the damage occurs but your character does not move the distance expected.