ecommerce is a tricky one. Trust is such a major factor, people wont buy if they dont trust your site or your brand. In your interviews, try to hone in on this. Are there any alarm bells that would potentially stop them from buying?
Maybe it’s the delivery cost, bad product shots, or just the price is not right? There could be any number of reasons why they’re not buying and some of them wont even be UX related. Maybe the traffic is the wrong traffic? Go through the analytics with a fine tooth comb and understand their marketing fully so you have a better understanding of where their traffic is coming from and if the marketing matches with the website.