Just got a "Your last flag was declined - please review it before flagging this post!" caution while flagging this as not-an-answer, which then led me to the flag-summary page, where I found a couple older declined not-an-answer flags with the note "flags should only be used to make moderators aware of content that requires their intervention".
FWIW I have a lot more not-an-answer flags marked as helpful, so it doesn't sound like I completely misunderstood the purpose of the not-an-answer flag, yet I am puzzled by these declined cases, as well as the reference to moderator intervention.
Related questions were asked before, for example «Not an answer» / «very low quality» flags, but I didn't find a definitive answer to when is it (in?)appropriate to exercise the not-an-answer flag, or what the difference is vs. VLQ in terms of moderator involvement/overhead.