The approver's user record has no email address configured in the Email field, preventing notification delivery. (high likelihood)
The approval notification record has conditions that evaluate to false, blocking notification from being sent to the approver. (high likelihood)
Email filters or notification filters are preventing the notification from being delivered to the approver. (medium likelihood)
The approval workflow activity is misconfigured with incorrect approver assignment or notification settings. (medium likelihood)
Email system configuration issues or SMTP server problems are preventing outbound notification delivery. (low likelihood)