Checkout failures are rarely one generic problem. A spinning button, a rejected payment, a blank response and an order that is created but never confirmed each point to a different part of the request path.
Start with the exact symptom
Record whether the failure happens before payment, during payment authorization, after the gateway returns, or after the order exists in WooCommerce. Note whether it affects all users, one browser, one payment method or logged-in users only.
Check the browser before changing PHP
JavaScript errors can prevent checkout requests from being submitted correctly. Inspect the browser console and network requests. A front-end error introduced by a theme, analytics script or extension can look like a payment failure even when the gateway is healthy.
Check WooCommerce logs and gateway responses
Payment extensions commonly record request or response details in WooCommerce logs. Look for authentication errors, webhook failures, rejected parameters and timeouts. Do not copy secret keys or full customer data into public tickets.
Exclude cache and optimization from checkout
Cart, checkout and account flows should not be treated like ordinary anonymous pages. Page caching, aggressive script delay or session interference can break dynamic state. Review cache exclusions before blaming the payment provider.
Test conflicts methodically
A staging copy is the safest place to switch to a default theme and disable non-essential extensions. Change one boundary at a time, because a successful checkout after disabling twenty plugins does not identify which dependency caused the problem.
Watch for server-side failures after the order is created
An order can be stored successfully and still fail during stock updates, emails, webhooks, subscriptions or third-party synchronization. Check PHP and WooCommerce logs around the same timestamp as the test order.
Do not repeatedly test live payments without a plan
Use gateway sandbox modes when available, keep test order records clear, and understand how the gateway handles authorizations, refunds and duplicate attempts.