Those of you who are Carnage specced may have noticed this, especially if you have done many operations. I first noticed it on the world boss on Belsavis, and then confirmed it later against Karagga in Karagga's Palace and on Graj in the Eternity Vault.
Ataru Form is not affected by increased hitbox size. A larger enemy typically has a larger 'hitbox' at which range is calculated; for these large enemies, being in melee range is not calculated from their center but from their edges. All melee attacks work this way on these enemies (Want to see? Go fight a Wild Rancor in the Belsavis Bonus Area. Other enemies are also large enough and easy to get to but this is the one I most quickly remember). Even Ranged attacks have a larger range due to a boss's increased hitbox. Essentially, it is expected that every form of attack is affected by the hitbox size.
The proc from Ataru Form, however, is not. Against a larger enemy, being in range for your melee attacks does not mean you are in range for Ataru Form. This is extremely easy to test. Stand at the outer edge of a giant enemy and use Massacre. You'll attack him as normal, deal damage, spend rage, etc. But you won't get your Ataru Form proc, even though it is supposed to be guaranteed. In fact, if you continue hitting the mob at this range, you'll find that you never get an Ataru Form proc. Ever.
"Big Deal," some of you may say. "Just stand closer." While, at the moment, that is obviously the only recourse against the bug (and since Ataru Form says nothing in the tooltip about it functioning different from every other single attack in the game, I assume it is indeed a bug and not a feature), it isn't sufficient and doesn't mean the bug should not be taken seriously. Against a boss like Karagga, being in range for Ataru Form can mean being required to stand in deadly, deadly fire. And when soloing, large enemies cannot even be within range. As soon as you move closer to them, they move back a step to be at the edge of their hitbox as well. This is easily demonstrated by the Wild Rancor mobs on the Belsavis Bonus Area. It is impossible to hit these enemies with Ataru Form.
And being unable to get Ataru procs gimps the Carnage spec immeasurably, a spec that is already (by some metrics) inferior damage-wise to Rage and especially Annihilation.
This is an easily reproducible bug that needs to be better known. Watch out if you plan on going Carnage, and Bioware: you need to get on this.
Ive seen people post on this, Are you sure its a bug? Have you tried zooming all the way out to see if the Ataru proc is actually showing up towards the top of the mob? It could be you just cant see the number.