The actual reason that it takes Willow until Buffy the Vampire Slayer season 6 to release Amy from her rat spell is far more tragic than it seems, and truly demonstrates Willow's descent during the season. One of the first witches introduced in Buffy the Vampire Slayer, Amy spends much of the show trapped in the body of a rat. But Willow, despite showing immense power and ability throughout seasons 5 and 6, seemingly waits until it suits her to save Amy.
Amy Madison was Buffy and Willow's fellow student at Sunnydale High. Introduced way back in season 1, episode 3, "Witch," Amy became a semi-regular character in the early seasons. Until Buffy the Vampire Slayer, season 3, episode 11, "Gingerbread," when an angry mob confronts Amy (along with Buffy and Willow), and Amy escapes by turning herself into a rat. Buffy and Willow attempt to reverse the spell, but are unsuccessful. Resultantly, Amy, in rodent form, spends the next few years living in a cage in Willow's bedroom. That is until Buffy the Vampire Slayer season 6.
Willow Released Amy From The Rat In Buffy Season 6 Because Of Tara
In Buffy the Vampire Slayer season 6, episode 9, "Smashed," Willow reverses Amy's spell. It is presented as if Willow suddenly realizes the solution, but Willow's incentives are much more selfish. Willow is motivated by the loss of Tara, her magical and romantic partner who leaves her for indulging in too much magic. When Willow (played by Alyson Hannigan) wants magical company, with a witch more supportive of her magical exploits, Willow turns Amy human quite easily. Either Willow forgot about Amy until it suited her, or she didn't dedicate herself to the situation because she was more interested in the magic she was becoming addicted to.
Willow's dependency on Magic is used as a metaphor for drugs throughout Buffy the Vampire Slayer season 6, in which Willow herself becomes the season's Big Bad. This was Willow's primary motivation in rescuing Amy. Willow wanted somebody around who would encourage her behavior, which Amy quickly does. When Amy later gives a magic-sober Willow some of her power, it cements Amy as the metaphor for Willow's out-of-control behavior and dependence on dark magics in the overarching magic-as-drugs metaphor of Buffy the Vampire Slayer season 6. This is why a recovering Willow later cuts Amy out of her life.
How Willow Could Have Saved Amy Sooner
Willow clearly had the requisite power and knowledge to rescue Amy already. Only eight episodes prior, in Buffy the Vampire Slayer, season 6, episode 1, "Bargaining Part 1," Willow raises Buffy from the dead, a magical accomplishment far more complex. With the final season revealing that Willow is a goddess, not to mention the many impressive feats which have implied so over the years, Willow had the power to change Amy back much sooner. In fact, in Buffy the Vampire Slayer season 4, episode 9, "Something Blue," Willow briefly restores Amy to human form by accident. Unfortunately, Willow does not realize this before inadvertently turning her back into a rat.
Granted, Willow did not appreciate the extent of her powers, not until she resurrected Buffy in season 6. But whether Willow was aware of how to save Amy or not, she clearly already could, and the four-year wait only happened because Willow's descent into black magic and addiction blinded her true, altruistic self. It is therefore the perfect precursor to Willow ultimately becoming the villain of Buffy the Vampire Slayer season 6.