Bo here from Shopify Support!
So sorry to hear that you are facing this issue. When it comes to abandoned carts it can boil down to a number of things. When a merchant gets in touch when facing an issue like this I always start by trying to replicate the issue the customer experienced. Take a look at your abandoned checkouts and click into the latest one. Take note of the products they tried to purchase and where they were trying to ship to. You will then go to your storefront and replicate that order. Add the same items to your carts and try to ship to the same country they tried to checkout to doing so will show you where in the checkout process they got to and what possible error message the received.
Generally speaking, it is usually one of the following:
Please do let me know how this goes and if you cannot figure out what the issue is, please do provide a link to your store so I can replicate the error on my side!
All the Best,