Autopsy: the offer that worked, then quietly stopped

Autopsy: the offer that worked, then quietly stopped
A steady performer. Same traffic, same creative, same page. Over a few weeks the results slid, and the obvious explanation — fatigue — was accepted and the creative refreshed. It did not help.
The actual causes, found in this order:
— The advertiser had changed their own flow. An extra step appeared after the click, invisible from our side, and it was eating people.
— Approval criteria had tightened. The same volume of actions was submitted, fewer were accepted, and nobody mentioned it because nobody had to.
— Competition had increased on the same placements, so the slice of audience still seeing it was a worse slice than before.
Only the third was on our side of the line.
The lesson: when a stable thing decays, check what changed outside your account before changing anything inside it. Walk the destination yourself, all the way through, at intervals.
Fatigue is what people say when they have not looked.