Skeletal Ramsters can rarely stop attacking spontaneously, even when they have valid targets . Until given a Direct Command or a new wave starts, they won't fire. Unlike other bugs of the sort, Skeletal Ramsters do not give a visual indication that they've stopped, like animating with no special effects. They simply go idle. Oddly enough, if multiple Ramsters have ceased attacking, giving a single Direct Command to any of them will restart them all.
Still attempting to nail down a reliable reproduction method. Every time this or similar bugs have occurred, it's always on the Dark Cleft lanes of Forgotten Ruins, and the enemy is a melee enemy that has gotten close enough to engage my barricades.
I realize a screenshot doesn't necessarily show much, but my girlfriend took a screenshot of the bug occurring. Note that the Orcs are hammering my Arcane Barriers, despite the fact that my Ramsters are strong enough to one-shot any enemy on the map before they can even reach the Fissure of Embermount field.