How to understand the cause of an error in placing an order in an online store?

Question

Software testing

Friends rarely but still receive complaints from customers that they have placed an order (confirmed the order in the basket and entered data), the page has been updated, but notifications about the order number or email and SMS notifications have not arrived.
There are such appeals once a month, but we cannot catch this bug by the hand.
Tell me how you can test or identify a bug due to which a failure occurs.
Moreover, in the metric, goals are set for visitors to the basket and those who place an order
. Maybe there is a methodology with the help of which errors or bugs are detected

Answer 1

Add detailed output to the log for each stage of the order, including intermediate ones.
And study the logs