The Animate Blastbones ult summons 3 blastbones, but because you don't target an enemy, the blastbones (seemingly) choose a target at random. However, maybe 1 in 20 or 1 in 30 casts, the blastbones won't choose any target at all, even if an enemy is clearly in sight and close by. When this happens, the ult is wasted and they expire without dealing damage. Please look into the targeting of these 3 blastbones, or consider allowing necro pets to be commandable with the Command Pet keybind.