That usually means auditing the mobile-specific tracking setup, especially pixel or SDK events tied to purchases. If clicks are still recorded but conversions vanish, it could be missing or duplicated events on iOS, broken app-to-web attribution, or UTMs being stripped on mobile browsers. I'd compare GA4 or ad platform conversion counts against backend sales to see if gap is reporting or real. Segmenting by device and age can reveal if a tag only fails for certain landing pages or app versions. Fixing that tracking layer restores accurate ROAS before any media optimization decisions are made.