Search terms drift
Matching loosens, new products change what you match, and a negative list set in March is irrelevant by September.
Creative fatigues
CTR on a previously strong asset declines slowly enough that nobody notices until performance does.
Feeds rot
New products arrive with supplier titles, discontinued ones linger, prices drift out of sync.
Tracking breaks silently
Theme updates and new apps remove tags. Conversions fall, the bidding pulls back, and it reads as a market problem.
Targets stop matching the business
Costs change, margins change, the target ROAS set a year ago does not.
None of these are dramatic. All of them compound, which is why "we set it up well" is not the same as "it is being run".