The Luminark of Hysh is very finicky about aiming it's shots. It's often shown to be "obstructed" for no clear reason, even though it's actually quite a tall model. It won't shoot in circumstances where it ought to be able to.
Try shooting the Luminark in various situations.
Details of any solutions that have already been attempted:
Moving the Luminark around the map and trying from different angles helps. Even if you are moving around a flat plain and nothing seems to be different it can make a difference between being obstructed or not, which is senseless.