This has to do with Cycle of Elements and how it interacts with Harmony of Elements, and killing mobs too fast.

What I mean is that if you kill a mob with a spell that will trigger multiple Cycle of Elements (and Harmony) buffs (like Elemental Burst), sometimes some of the effects do not go onto the mage because only some of the damage was applied.

For example, imagine a mob has 1000HP, and your Elemental Burst does 500 damage PER ELEMENT to it, usually doing 2000 dmg total, but in 4 pieces of damage. What I have observed is that ALL of your "Harmony of Elements" buffs get applied, but only SOME of the "Cycle of Elements" buffs do. I'm guessing the flags are different, but basically because only two of the elemental damage was done, it only triggers those two cycles.

THIS WILL NOT HAPPEN ON A PRACTICE DUMMY. This is because it never dies, and thus all effects always take effect! Thus testing may have missed this.

This is most visible in large number of small-HP mobs (invasions, or sometimes IAs). It has become a new habit of mine to lead with Elemental Burst, and then spam Crystalline Missiles, since it is then Insta-cast (Cycle of Fire) and AE (Cycle of Air). But sometimes the insta-cast doesn't happen. I have observed this ONLY happens if I use Elemental Burst on a low-HP mob, killing it, then the other cycles don't always get applied correctly, but I definitely have 4 "Harmony of Elements" buffs.

The lack of the Fire cycle is REALLY visible on Crystalline Missiles, as you can move if you have it and cast (insta-cast) but without it, it just fails.

Anybody else have this problem? It's possible (though unlikely) that it's interacting (badly) with Epidemic from Necro, but I'm 99% sure that's not it. It only seems to happen with killing mobs with Elemental Burst, but I'm not 100% sure. More investigation needed, but I'm fairly sure of the cause. If a mob takes the entire hit and doesn't die, I've never had the issue.