Not receiving some direct message email notifications

Hi,

I've noticed that sometimes I don't receive direct message notifications over email. Today for example, I've received 5 Direct Messages and only 2 notifications. 2 of the direct messages are from the same person, and I had a notification for only one of those.

Any idea why I'm having this problem? I've checked my junk mail folder and my trash etc. I'm using Google domain account email.