In Bleach's Hueco Mundo arc, Hollows, the monstrous souls of the undead, get a devastating upgrade. Going from masked monsters to unmasked warriors, the Arrancar became a palpable threat in the series from that point on. However, even without their masks, the Arrancar could never escape the truth of their empty existences.
After the Soul Society arc, the traitor Reaper Captain Sosuke Aizen ventured to the desert world of Hueco Mundo to recruit the most powerful Hollows and, with the musical power of the Hogyoku, mutate them into unmasked Arrancars to fight for him. But using Arrcanars to do his dirty work was always destined to fail, by the very nature of what Hollows are.
Sosuke Aizen presented himself as the savior of all Hollows, but in reality, he only wanted to manipulate them. A number of Arrancars even saw through his act, with Nnoitora Gilga, the 5th Espada, once commenting that there is truly no salvation for Hollows or Arrancars, and that not even Aizen could change that fact. Nnoitora wound up being right, though he didn't exactly spread the word. So, instead, Aizen gave the Arrancars false hope that they could find purpose in their lives if they fought for him, only for him to treat their lives like disposable chess pieces.
Hollows are named after the hole in their chest where their heart should be, and they try in vain to fill that void by devouring spirits, usually those of humans. This hunger can never be sated, however, and Hollow evolution from Gillian to Vasto Lorde just strengthens a Hollow without freeing it of this cycle. Grimmjow Jaegerjaques, the 6th Espada, once dreamed of becoming a Vasto Lorde king, but he never could, not even with the extra strength he gained from serving Sosuke Aizen. Even Barragan Luisenbarn, who actually ruled Hueco Mundo as king, found that his success was pointless. He was simply the strongest, and nothing more.
For a spirit, true happiness can only be achieved by moving on to the Soul Society, and this includes purified. But Arrancar are simply too far gone and too complex to move on as they are. Sosuke Aizen couldn't -- and wouldn't if he could -- change the fundamental nature of what an Arrancar is, the very thing that holds them down.
Beyond that, Aizen's Arrancar army was hardly an army at all. Aizen tried to create the Hollow version of the Gotei 13, but with very mixed results. He gave his Arrancars a white and black uniform, the inverse of Soul Reaper robes, and loosely organized them with ranks and brands. The 10 strongest formed the Espadas, and each Espada had lower-ranked Arrancars serve under them as a squad of Fracciones, with Aizen at the head of everything. However, while some Arrancars like Loly and Zommari Rureaux were greatly loyal to Aizen, many others were only loyal to each other, even in the higher ranks.
In hindsight, it was foolish to presume that all Arrnacars would fall in line behind a total outsider, and indeed, most Arrancars were using Aizen's project for their own ends. Tier Halibel, for example, had three Fracciones who stayed with her for mutual protection, and she used her power as an Espada to keep them even safer. The same is true of Grimmjow's six-man squad, and Grimmjow even openly disobeyed Aizen, and his five minions followed Grimmjow without question. Most of all, ex-king Barragan deeply resented Aizen, and his six Fracciones were most loyal to him, and not Aizen. Those Fracciones only acknowledged Barragan as their king, and they hardly even mentioned Aizen at all.
Aizen was essentially trying to tame wild animals into attack dogs, though it's possible that he knew this all along. Hollows cannot be saved, nor would they replace their friends and pack leaders with Aizen in their minds. Instead, Aizen's Arrancar army really only bought Aizen time enough to slow down and wear out the Gotei 13's warriors before he himself started fighting. In a way, Aizen was really just humoring his allies and his enemies alike with the Arrancar army, knowing full well how futile it all was. Then again, Aizen is the type to deceive, use and betray people in cold blood, so it is hardly surprising. He would give a million Hollows false hope if it meant buying just one second of time to realize his grand ambitions.
