Slitherine

After a break of about three years I have started playing SOW Gettysburg again. I am currently playing the third tutorial about artillery. I had the artillery shoot at infantry but most of the shots were air bursts above them. In the info box the icon of "Shooting Over" was showing. Is this just a function that the infantry are at a range that the barrels cannot be lowered enough to hit? Is there a command I can use to tell the infantry to lower their barrels or is this handled by the AI? Thanks.