The audit before the experiments
Most CRO failures start with running experiments on a site that has fundamental problems. The fix is to audit before testing.
The conversion-killing issues that show up most often: slow mobile load speed (anything over 3.5 seconds on 4G is a problem), product photography that does not communicate the product properly, missing or weak product descriptions, no reviews on product pages, hidden shipping costs revealed only at checkout, complicated returns information that makes buyers hesitate, no size guide or unhelpful size guide on fashion, and a checkout flow that requires account creation before purchase.
Fix these structural issues first. Each one alone can lift conversion 10 to 30 percent. Compounded, the difference between a site with all of these wrong and a site with all of these right is significant.