Concurrent Usage

Is it just me, or is only one person allowed to use Shock at a time? Earlier this weekend, I just tried using Shock at the same time as a friend and noticed that the mobs were getting hit only once by his Shock. It looked like mine was having absolutely no effect at all. Is this a bug, or is this happening to everyone too?

Supposedly shock is having issues. It doesn't seem to see Ghasts at all (it won't attack them) and I've heard it doesn't stun some advanced heavy stander mobs that it was supposed to.