You spend months testing creative. Hooks, formats, audiences, offers. Then you take every expensive click that work earns, and you push it through a funnel you have never once questioned. Sometimes that is a raw product page. Sometimes a generic listicle into a generic PDP. Sometimes a homepage. Almost always a path that was not designed for the specific visitor who just bought into the specific ad.
The leak is not your ad spend. It is the funnel. The architecture between the click and the conversion is the highest-leverage and least-tested asset you own. It is also the cheapest to rebuild and the most expensive to ignore.