There are some posts which I have flagged and some action is genuinely required for those posts. I would say they are recent, but there were even more recent flags raised by me which were quickly acknowledged. But a few are still unattended/pending. I am not in a hurry to get them reviewed, but I just want to know that how do I again bring them to moderators' attention. Is there any way to re-flag a post?
You cannot "re-flag" a post, in particular not if you are talking about an "off-topic" flag, since users are allowed to flag (or vote) to close and reopen each post exactly once.
Furthermore, off-topic flags are generally not handled specifically by moderators, but by the user-driven close review queue. So while sometimes you may get "lucky" because a moderator acts on the post, generally you should not expect moderator attention for any off-topic flags. The processing time for these flags highly depends on how many users are currently reviewing them and whether the reviews are unanimous or not.