Facebook's delivery system puts ad sets through a "Learning Phase" while their system optimizes for the best targeting for the ad set.
In Consumer Acquisition's Advanced Reporting, you can get a breakdown of what phase each ad set is in.
- To see this information, go to Advanced Reporting, select the Data menu, and under Settings, check Learning Phase.
The report shows what state your ad sets are currently in.
Note: If you change your settings a lot, the current setting may have no bearing on past data, as a result.
You can read more about the Learning Phase in Facebook's Business Help Center:
Facebook Learning Phase