There are a few reasons as to why we may not have collected a specific post you were hoping to see in your Brand24 account.
Some of the likely reasons include:
Public post was published on a personal profile (not Facebook Page)
Post was published in a Facebook group
Public post previously held different privacy settings (i.e. friends only)
Public post is set to be visible only for specific countries
By far, the most popular reason for missing mentions is that said mention(s) come from personal profiles.
It is worth emphasizing that due to privacy-related changes, Facebook excludes the ability to collect mentions from personal profiles.