Spent a couple of hours tearing my hair out over the Facebook insights API this week. Say you have a Facebook page with an ID of 173304932707127, and a post on that page with ID 497058920331725. The Facebook Insights API docs would lead you to believe that you can query for post level details with just
/497058920331725/insights as the URL. Instead, any query at that base URL just returns an empty data property:
The fact that just
/497058920331725 correctly returns the meta data about the page would further strengthen your belief that there just is no insights data for this post.
However, though it is mentioned nowhere in the docs, you in fact need to hit
/173304932707127_497058920331725/insights to retreive insights data for that post.