This is broad enough that covers a lot of reasons why your conversion rate is low but your add to cart numbers looks fine. For the brand I was consulting, I didn’t know for the longest time they only shipped in the US since I’m based in LA so whenever I tested their checkout, it worked fine. We found the problem after looking into their GA traffic source and found that 40% of their traffic came from countries outside of the US. With their massive social media following, it had exposure to Canada, India, Singapore, Australia, and other countries with people that would love to buy but just couldn’t.
We found our ‘true’ CVR by isolating only the country we ship to and looked at the metrics pertaining only to those visitors. It was healthy and everything else looked fine.
This reason also covers payment methods like enabling PayPal. For brands that don’t have shop pay, Apple Pay, or PayPal, customers that don’t have their card around just wouldn’t be able to check out, which will lead to them abandoning the cart.