What types of targets is the ship fighting against? Frigates, or something larger? What "ai hints" did you specify in the spreadsheet for it? DO_NOT_AIM sounds like one you might need, if the missiles are guided (and don't need to be fired directly at the target). You probably don't want STRIKE.
Ahh, also - did you set the "range" on the weapon, in the spreadsheet? For missiles, even though they are governed by speed and flight time, the AI uses the "range" column as a hint for when to fire the weapon.
How much ammo do the missiles have? If it's low, the AI will be very conservative about firing them, waiting for a good opportunity (shields down if the missiles are HE, shields up if they're kinetic, etc).
Edit: the Pilum LRM sounds like the best missile to copy from for your purposes.