or why one of the investigators didn't believe the story
Is there a reliable source that a particular investigator didn't believe Zimmerman's story, or is this based on the fact that the police report was merely titled "manslaughter"?
There a Serino or Seriano, or something like that affidavit, that disagrees with the decision to not at least hold, and maybe to charge Zimmerman. This is separate from the "what pigeonhole is this investigation in" entry on the initial investigation forms.