There is an issue with the Ballista, where the bolts cannot hit targets above or below the summon.
Since the Ballista does not aim vertically, this forces the player to seek out a plateau and deploy the ballistae there, to which the enemies have to be lured by themselves, as Ballistae won’t be targeted as often as players.
This issue is easily reproducible on the Black Sun timeline, where the trail to the Dark Ascent leads through a long serpentine slope: the Ballista’s lack of aim leads to constantly missed/absorbed shots by the terrain: Issue on Dark Ascent.
I found Llama8’s post from last year, where the exact same issue is marked as resolved, but I meet it from time to time again: Ballistae don’t hit mobs on a lower level than them.