Notification is configured and seems to work correctly for most of the customers, but for some of the customers the notification cannot be found when searching by the notification subscription and the customers e-mail address in Home > System > Logs.
The customer may have changed his e-mail address.
If an e-mail address has ever been changed is only possible to find out from the database:
select "BillEmail","UserArc", "DateArc"::abstime from "AccountArc" where "AccountID" = <ID> order by "DateArc"
<ID> is to be substituted with the id of account. You will get the results
BillEmail - an e-mail address UserArc - the user who made changes DateArc - when the change was made
If an account in question is of personal type (not business account), then "PersEmail" instead of "BillEmail" to be checked:
select "PersEmail","UserArc", "DateArc"::abstime from "AccountArc" where "AccountID" = <ID> order by "DateArc"
After getting the previous e-mail address, recheck the notification in Home > System > Logs.