When server lag spike hits you, anything can happen. Ultimates not firing is only a minor part of the problem. I've had bar swap not working, potions not activating, block not registering, stamina being drained to zero while just standing still doing nothing, abilities and light attacks not going off, taunts being ignored, my character dying to attacks that I dodged several seconds earlier, and various other weird glitches.

Your instance of vMA probably spawned on an overloaded node on their server cluster. All you can do right now is hope for better luck next time. For the next update, they have announced some improvements to server load balancing for trials, but we'll see if it works, and if so, whether it also applies to arenas.