I had this happen yesterday too, several times in one game, with 2 different HQ's.
In the tutorial mission, I followed the manual's instructions and was able to fire mortars from over the ridge.
In my game yesterday, on two occasions I had HQ's with attached mortars that wouldn't work. One example, HQ was ~200 meters from a MG bunker. When I use LOS tool for the HQ, it showed a blue line to the bunker. The mortar unit, in command, would give a red/black line to the bunker and say "No line of sight". I could draw the target back about 5m and get an area target, but never could gain a target on the bunker itself.
And yes, both times the HQ was not hiding, standing in open ground.
:edit: Is it possible that the quality of the HQ and/or mortar team would determine if spotter indirect fire is possible?