Starting August 5, 2019, U.S Customs and Border Protection have made a change to ESTA's which will require some people to re-apply.
"The ESTA website will be updated on August 5, 2019. All ESTA applications that are in a pending payment status at the time of the update will be archived. Applicants impacted will have to reapply after the update is complete. Citizens of participating Visa Waiver Program (VWP) countries traveling to the United States are strongly encouraged to apply for an ESTA at the time of booking their trip and no later than 72 hours prior to departure."
If you have recently completed an ESTA, or have a previously completed ESTA, and the payment is still "pending" you must reapply for a new ESTA as your application will be archived. Depending on what time they do the purge on the 5th August, I would make a note to check on the 6th August if you think your ESTA was still pending.
While for most of us making sure our ESTA says "approved" is something we give our full concentration too, there are many UK holidaymakers who simply think they need to apply and that's it, and don't check the status (or even complete the payment). If you know someone travelling to the US soon, make sure you get them to check it went through fully.