Why updates to ticket, containing public replies not being sent to end-users?
In Zendesk Support, emails are sent via Triggers and by default, the trigger responsible for sending your replies is called: "Notify requester of comment update".
Make sure that this trigger is active in your account by going to Admin () > Business Rules > Triggers and checking the "active" tab. If you find it in your "inactive" tab, please reactivate it like this:
If however, this trigger is correctly active in your account, please contact us at firstname.lastname@example.org with more information and ticket examples to help us investigate further on this issue.
For more information on the default triggers and their roles in your account, you can consult this help article: About the Support default triggers.