The signal
CTR declining on an asset that previously performed, with everything else stable. That is fatigue.
Which channels care
Demand Gen and YouTube most — the asset is the campaign. Search far less, because relevance does more work than novelty. Shopping barely, unless the product image is the problem.
What to change
The hook first. Most creative fails in the first two seconds, not in the offer. Produce variants of the idea that worked rather than starting again.