Games : Why do targeted abilities sometimes fail to cast?

I'm on my second playthrough and I've noticed that some area of effect abilities sometimes fail to cast. For example, the Demolitionist's Canister Bomb ability or the Arcanist's Trozan's Sky Shard ability will sometimes not be cast when hitting the key mapped to the ability. I have noticed that it seems you can't target enemies behind walls with Trozan's Sky Shard, although I was able to target a zombie that was beyond a line of trees. Is this simply a bug?