fire value for mortars

[Mowgli]

Just to rule out some of the potential causes:

Is the fire occuring in your turn or is it opportunity fire (*0.5 effectiveness!, manual p. 47)
Is the spotting unit detached (i.e. outside its HQ's nominal command range; *0.5 effectiveness!, manual p. 41)?
The fatigue of the firing mortar unit is good, I suppose?

@ alternate fire resolution (mentioned by Green, manual p. 106)
Note that the quote above says that the "alternate fire resolution" does NOT apply to mortars. Also, one would expect that (against single targets), the alternate fire rule would INCREASE firepower, not lower it (if there is only one target, the full *2 firepower would hit that target, not be spread out over many targets according to their strength). This also helps explain why only mortars seem to be so weak (they do not get their fire values doubled like other indirect assets...). But of course it cannot explain the actual fire values observed in the game, which really seem to be unexpectedly low...

@ stack fire/density modifier (mentioned by wololoh, manual p. 46, 106)
The rules seem to indicate that this factor can only increase, not decrease firepower. It doesn't say that firepower gets decreased if firing at a hex that is below half its stacking limit. I suppose that this is needed because of the alternate fire resolution: If there was no bonus for firing at "full" hexes, then the fire of indirect assets would get spread out too thinly over too many units. So you need to give arty a boost.

But maybe the manual is indeed misleading and it's the other way round (a malus for firing at "empty" hexes, rather than a bonus for firing at "full" hexes, which naturally hurts mortars who do not benefit from the "alt res"). Indeed that's the only explanation I can think of.

I also found this obscure note in the manual (p. 106):
"Total indirect fire on any one hex per turn limited to 1.5 max stacking value."