From: Tucson, AZ
The observer you are using to shift the fire in is too far back. Use a closer observer, like the platoon leader. For some reason the game equates the distance from the FO as the weapon range while calling fire. If the FO is 46 hexes from the target of an 81mm mortar, then you will get the out of range message, even if the mortar itself is closer to the target.
I wondered about this for some time. But after this teory was presented, I tested it and found the results to bear it out. It's the best explanation I've come across so far.
"Always mystify, mislead, and surprise the enemy, if possible. "
- Stonewall Jackson