When a customer says they never heard back, the reply usually did leave the helpdesk. Check these in order - the first one is by far the most common, and it is not a configuration problem at all.
A reply goes to the customer. A note stays inside the ticket for your team and the customer never sees it. Both appear in the same thread, which is exactly why this is so easy to do under pressure. Open the ticket and look at how the message you sent is labelled. If it is a note, the customer is still waiting.
Ask the customer to check their spam or junk folder before you change anything. If it is there, have them add your support address to their safe senders, and consider asking corporate customers to whitelist the Raiseaticket sending domains so it does not recur for their whole company.
Notifications are configured per action, so it is possible for ticket-created mail to work while reply notifications are disabled. Check the notification settings for the rule that covers agent replies, rather than assuming that because one type of mail arrives, all of them do.
Tickets raised on someone's behalf, or from a shared alias, can carry a contact address that nobody monitors. Check the requester on the ticket against the address the customer expects you to answer.
By default replies leave from the Raiseaticket domain rather than yours. Strict corporate filters sometimes distrust mail that says it is from your company but was sent by another domain, and quarantine it without telling anyone. The durable fix is to connect your own Office 365 or Google Workspace mailbox, so replies are sent from your own address and pass your customers' checks the way your ordinary mail does. That is a premium feature.
Whatever you change, prove it with a real test: reply to a ticket raised from an outside address you control, and confirm the message arrives in the inbox rather than the spam folder. Do not close the customer's ticket until you have seen a reply land.