Though it differs from map to map, if you do consecutive casts (Ranges from 50-100) of explosion, the skill becomes a graphic of this: It has none of those explosion particles, and does no damage to nearby mobs.
Which means you've probably reached the maximum amount of consecutive casts allowed, after 100 or so consecutive casts your skills won't do anything until you move a bit. It's not a bug though, it's intended.