Category Blocking
If you unblock one app or website, it may still stay blocked when it belongs to a blocked category.
For example, if a blocking session blocks the Social category, then Facebook is blocked by that category. If you only unblock Facebook, the Social category can still continue blocking it.

What To Do
To access the app or website, unblock the category too, or edit the blocking session that contains the category.
If you want to unblock specific apps or websites one at a time, consider selecting individual apps and websites in your blocking session instead of selecting the full category.