The diagnostic
Compare CTR and CPC against your own history — if they are stable and conversion rate fell, the problem arrived after the click.
Usual causes
- Page speed, especially on mobile.
- Shipping cost revealed late.
- Out of stock variants.
- A theme or app change that broke something.
- New reviews.
Why it gets blamed on ads
Because the ad account is where the number is visible. The account reports the symptom; the page is the cause.