Has anyone experienced member receipts not being sent out all the time when sending from the back end of a membership record?
Likely you'll find the answers you need in the CiviCRM debug log. Also check your server's mail logs to confirm what's actually happening, so you can identify whether there's an issue with the email being generated or delivered.


Where should one look for logs when debugging a new problem?
CiviCRM wiki: Debugging for developers


It's possibly an issue with a customised message template not being valid Smarty - we've seen this affect Smarty templates before, which use { / } to denote template markup and are not validated when edited through CiviCRM.

As a result it's very easy for admin users to unknowingly include CSS or JS in the message templates, not knowing the template will fail when executed in context of a system email.