Starlight has been incredibly derpy for a long time, I think one of the issues is that you have to set it to hold fire if you want to give it commands, otherwise it will constantly ignore your commands for some reason.
Also since (like most things) it's basically programmed to miss radar blobs, that inaccuracy means it fixates on nothing half the time. It seems like an attempt was made to avoid this by having the beam move around constantly but the result is pretty crappy and I would guess that crappy coding is why it ignores attack commands sometimes but just guessing.