Magic Bomb doesn't deal damage

Sometimes when I use magic bomb I don’t hit the mob even though the mob is clearly within range… https://gyazo.com/81c1433d6e810fde0fd00175593e6dab

I don’t consume mana when this happens, but I still get the skill cooldown which is annoying. My magic bombs deal ~350-400 but all the hits I dealt to the spider were from melee attacks (the 390 was from a critical hit from my basic attack). This happens often and it’s annoying because I don’t kill mobs as fast…

The other time this happens is when you use two skills at near the same time (the second skill doesn’t consume mana or deal damage), but that doesn’t seem to be what happened here…