The most common reason for this is that your hosting server is not configured to use PHP mail() function. Another reason is that many email service providers use a variety of tools to reduce email spam. Sometimes the email is sent out by WordPress, but it never even makes into spam folder of the destination.
That’s why we recommend using SMTP to send emails in your MainWP Dashboard. SMTP (Simple Mail Transfer Protocol) is the industry standard for sending emails. Unlike PHP mail function, SMTP uses proper authentication which increases email deliverability.
There is a variety of 3rd party plugins that can help you with resolving this issue. Here is a few: