Performance rarely falls off a cliff. It erodes, and the cause is usually creative fatigue, not the algorithm.
Start with frequency and thumb-stop rate. If the same people see the same ad five times, results drop no matter how clean the campaign structure is. Refresh the hook before you touch budgets or audiences.
Then test one variable at a time: the first three seconds, the offer, the proof. Keep a simple log of what you changed and what moved. After two or three cycles you will know which angle carries the account.
Finally, check the page the ad lands on. A great ad sending traffic to a slow or confusing page looks like an ad problem when it is a page problem.