Sophia here from Shopify. Welcome to the Shopify Community!
I'm sorry to hear that some of your customers have had issues with completing their purchases on your store. Luckily, we should be able to find additional details about each unique checkout experience to determine the cause of the issue for these customers.
To find the details about each attempted transaction, you can head to Orders > Abandoned checkouts in your admin and view the payment events from each abandoned checkout. Here, you should see a reason listed for the transaction failure in the History section if the customer entered their payment details. You can then click on that note to see more information from the payment gateway.
In some cases, there could be a general issue with the payment gateway on a store. However, if other customers are currently getting through with the same payment gateway, then we're likely looking at individual issues on the customers' end. In those cases, the error messages you see might relate to insufficient funds, a card that isn't enabled for online purchases, or an error with the customer's account (potentially the wrong address or funds on hold, for example).
Please let me know if the information you've found under Orders > Abandoned checkouts helps you to resolve the issue. If not, I'd be happy to send you an email for further investigation if there are any particular orders you're concerned about. We can absolutely continue to chat here as well, but keep in mind that we don't want to share your customers' details here in the Community thread as they need to be kept confidential.