Are you noticing performance fluctuations at the start of your campaigns or after making edits?
It's time to explore Facebook's Learning Phase.
After your ad is approved and delivery begins, Facebook needs to collect data in order to show your ads to the people who are most likely to complete your optimization event goals. An ad set needs about 50 optimization events after the start date, or since the last significant edit in order to complete the Learning Phase. Keep in mind, every time a significant edit is made, the Learning Phase resets! To ensure stable delivery, keep significant edits to a minimum unless necessary.
The following actions will cause the Learning Phase to reset:
Any change to targeting
Any change to ad creative
Any change to optimization event
Adding a new ad to your ad set
Pausing your ad set for 7 days or longer (the learning phase will reset once you un-pause the ad set)