Keep in mind that the caster LB's default location is the center of your screen, so if you are looking up at an enemy or across the battle area, the marker may not show up since it wouldn't be on the ground. It's best to point your camera at an angle towards the ground when using caster LB or just simply using a macro with the /ac "Limit Break" <t> command for it to center on the enemy you have targeted. (Macro will still work if you don't have a target but will need to be manually set)