Unfortunately, it is not always clear why a Craigslist ad has been flagged. The most common reason we have seen is not placing the ad in the appropriate Craigslist location/region. Typically, Craigslist wants the property to be listed in the region in which it is located, as opposed to any nearby regions.
This is Craigslist's own explanation of why ads get flagged: flags and community moderation
They have also published an article on how to finding your post online: how to find your post in the listings