JetPack subscription not sending emails when new posts are created

JetPack subscription not sending emails when new posts are created. I'm using the Blog Subscriptions (Jetpack) widget. One user subscribes the confirmation email is sent, but once a new blog post created it should send an email notification and it doesn't.

Only these plugins are enabled on a site:
Google Tag manager, Jetpack and wpDirAuth

It's working on other sites.
- Did check for a conflict;
- Enabled WordPress debugging there was no output;
- Server email's log doesn't show any errors for blocking emails

JetPack support team haven't responded yet.

Can't share a client's site due to confidentiality.

Please advise if you have some suggestions.