I believe that you have to turn the AI commander on for indirect fire pieces for them to automatically fire on targets seen and designated by an artillery commander.
That's one reason why I don't put them under the command of the AI.
I've often had the artillery commander stop indirect fire missions on targets I considered more important so that they could fire on targets that they had a direct line of sight to,and usually those targets weren't even worth a single mortar round let alone 5-6 and the AI will expend all of its mortar ammo to take out insignificant targets.
I don't give the AI carte blanche for important tasks such as indirect fire control,I personally want to control all indirect support assets.
I had one battle where I assigned a 2 gun 81mm mortar battery an indirect(area) fire mission on an enemy force that looked like 2 platoons worth of infantry,and I was really putting a hurting on them and then the fire stopped for some strange reason eventhough I knew the mortars had a lot more ammo to spare.So I zoomed the camera over to the mortars to see what the problem was and I see them firing on some individual soldiers that weren't even a threat,but they started firing on them anyways because the battery commander could actually see them directly.
Which is the main reason why I do not give indirect fire weapons AI command.
I also "don't" set up my indirect fire weapons in areas were they have a major line of sight on the battlefield,because the mortars will fire on targets that I wish them not to and they are usually targets that aren't worth the ammo expenditure.Which is why I usually put them behind a building or treeline with limited line of sight so that they only fire "indirectly" on the targets I feel warrant the ammo expense.
I mostly want my indirect fire units to concentrate on either large infantry formations or MG,mortar or IG locations.Sometimes I may use them to pin infantry in place while I wait for my off board artillery to start raining down before they move away from the Arty kill zone.