Your performance may fluctuate if your bids are set without taking into consideration the historical data Google Ads will use to serve your ads.
Example
If your campaign is using the Target CPA bidding strategy and the conversion action targeted has seen actual average CPAs of around $10 USD for the past 14 days.
If the set Target CPA is lower, you may see fluctuations more frequently. The reason for this is that the campaign may need to decrease spend periodically, depending on the CPA it is achieving, as it optimizes to ensure the achieved CPA is within that lower target on a monthly average basis.
What you can do
Smart bidding
For Smart Bidding, we recommend using historical data and choosing bids within a close range of the actual averages to minimize fluctuations in performance.
Need something else?
Check other common reasons why your campaign performance is fluctuating.