The reason the Grandes aren't in Season 2
At the end of "Unlocketing The Past, Part Two," as part of Griselle's plan, she and Gwen hide out in a shrink-proof bunker for three months. We don't know exactly how long the second season takes place after that. So, it's possible they're still inside it, which would also explain why their absence is never commented on. The series is also shown to stick by continuity, so it'd make sense for something like this to happen.- Griselle and Gwen return in Season 5, but don't explain where they've been. Griselle tells Polly she's been making an effort to be a better person, which may at least explain why she hasn't been attempting any schemes to get the locket.
The Grandes' role in Season 6
Griselle and Gwen's one scene in Season 5 was setting up them going back to being the main antagonists again in Season 6. Bringing Griselle's voice actor, Patricia Drake, back for just one scene would be a waste of time and money.- Griselle told Polly she's been making an attempt to be a better person, but she may relapse into her old ways once she discovers there are other forms of pockite.
Griselle lied about turning over a new leaf
It's really a cover as she's planning something else.Griselle is genuinely trying to be nicer and will join forces with Polly against a new enemy
Grunwalda returns as the new Big Bad.