For now, don't fire from slopes, make sure you're in range and don't give an out-of-range attack order, and make sure you only attempt to fire when it says it is no longer on cooldown (I think the UI bug is still there that allows you to try to fire whilst on cooldown, it doesn't let you but it can fuck it up).
The point is that I could not even make it fire once, no matter what range, slope, barragecooldown anything.