According to this answer by Shog9 on the Stack Overflow meta, the "very low quality" flag is available only within the first seven days of a posts lifetime.
The rationale is that serious problems, which the VLQ flag is for, are typically quickly detected and handled (by deletion or sometimes editing). If a post survives much longer, the problems probably aren't so huge that a VLQ flag is the right option.
How to deal with such "zombie" answers that somehow have made it through the review process, that keep collecting downvotes and that seem to lurk below our radar forever?
If they keep collecting downvotes, for users with at least 20k reputation, there is a "delete" link available which should be used on posts that really oughtn't to be on the site.
Note, however, that just being wrong is not grounds for deletion. If something posted as an answer doesn't address the question, go ahead and vote to delete it. Or flag it as "not an answer". If you have already flagged it as NAA, but the reviewers didn't agree, you can raise a custom moderator flag (explain that the post doesn't address the question, and hence should be deleted). But of course there is no guarantee that the moderators agree with the assessment that it doesn't address the question.
If it's just wrong, even terribly wrong, let it be covered in downvotes, but that's not reason to delete. If it's dangerously, misleadingly wrong, that can be a reason to delete.
If it's meaningless gibberish, it should have been dealt with earlier, but if it hasn't, a custom flag or a NAA flag are appropriate.
How can we kick them once again in the review queue in the hope of seeing them deleted?
If it already was in the "low quality" review queue, it doesn't enter that queue again, then you need three trusted users or a moderator to agree that it ought to be deleted.
