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.
Did some more testing alongside my GF, and I think I've pinned down the reason why this is occurring.
In each instance where this has occurred, I've set up my Ramsters so that the edge of their range grazes the enemy spawn. They have about the same range as a Javelin Thrower. So whenever Javelin Throwers come out of spawn, they stand on the very edge of the spawn barrier, and hurl javelins without moving. My Ramsters are detecting this as a target in range, but because he's standing on the edge of the spawn protection field, they're refusing to see him as a valid target and firing.
And they never think to check for other targets they can shoot at.
As a result, they're not shooting at anything, allowing other enemies to engage my barricades. If anything kills the offending Javelin Thrower, the Ramsters suddenly recognize that there are other valid targets in range and begin firing again. This is why the idle Ramsters restarted as a group once a Direct Command murdered the enemy that was clogging their AI routine.
See for yourself. I got multiple screenshots this time, and in each one, there's a Javelin Thrower at the edge of my Ramsters' range, standing on a spawn barrier.
Here's the bug occurring on Dark Cleft double-lane. I managed to get attack ranges to show.
Here's a shot that my GF got of it occurring on the single lane to the west.