new wiz spell mechanic question

[kilgore]

lvl 16. When casting bolts sometimes i see the bolt hit the target but it doesn't explode. Mana gets used but the target does not agro. It's as if target absorbs the spell and nothing happens but the mana is wasted. Sometimes happens a couple in a row on pulls. I also notice it when i chain cast my bolt as soon as cooldown ends.

Do i have to wait a sec after cooldown before re-casting? I suspect its just a little fickle. Thanks