Here's a tip when using on-map mortars. Sometimes these units will get it into their little heads that they know better than you what to target. For example I ordered this unit to area fire an enemy 150mm gun that was causing a lot of grief. However ten seconds into the turn it decided that trying to knock out a panzer was a better use of its talents:
This often happens if a mortar was given indirect fire orders and a direct fire target appears in its LOS.
To avoid having this happen just put a very narrow arc around the target unit or area, then the mortar will stay focused on that and won't be tempted by other targets.