High add to cart, low purchases. What usually causes this?

In your experience what’s the usual reason? Is it shipping cost surprise? Delivery time? missing payment options? trust signals? Or is add to cart just a vanity metric and I’m reading into it too much?

If you’ve fixed this before, what was the one change that actually moved the needle? I’m not asking for a list of 25 micro optimizations. I mean the thing that made you go oh wow, that was it.

Also do you track ATC seriously, or do you basically ignore it unless purchase rate moves too?