Playing heel is supposed to be easy. Unhampered by the moral restraints of their babyface counterparts, wrestling's bad guys can essentially get away with doing and saying whatever they want, so long as it gets them booed. It's much easier to get people to hate you than like you, and thus, the sport's villains are often said to have a much easier time getting over than the roster's heroes.
This may well be the case, but being an effective heel is a lot more difficult than grabbing a microphone and saying a few mean things each week. If the performer can't make the audience buy into their act, they'll draw no reaction at all, and if they're too affable, they'll be cheered in spite of their actions. Both represent failure in every heel's ultimate goal of drawing crowd heat, and WWE history is peppered with examples of both.
Some flopped because of the way they were presented by the company, while others just weren't skilled enough to flourish in their newfound spotlight. Either way, none of these heel runs generated lasting vitriol from the crowd, and each was eventually abandoned.
Aaron Aguilera initially served as one half of Los Conquistadores during an angle with Edge and Christian at the turn of the century, but was let go at the story's conclusion. It took him four years to make it back to WWE, but he eventually re-emerged as Jesus in 2004.
Carlito had just become United States Champion at the time, and was embroiled in a feud with the fast rising John Cena. Having taken the title in his first match company, Carlito enlisted Jesus as his bodyguard, leading to an ill-conceived angle that saw Aguilera kayfabe stab Cena in the kidney during an altercation at a Boston nightclub. This kept John off television for a month, but he eventually returned, first regaining his US Title, then facing Jesus at Armageddon.
Cena inevitably triumphed, but WWE literally wrote Jesus to stab another human being as part of the storyline, and he still couldn't get over. The audience were almost silent as he emerged for the Armageddon match: Jesus failed to connect on every conceivable level, and he was eventually released in 2005, having gone for back surgery immediately after the conclusion of the failed Cena feud.