You can see in this image that the user has triggered the event and was placed in the series. But there is no history of communication - where this email would normally appear.

In case anyone else struggles with this, Caijsa from Intercom was amazingly helpful and helped us solve this.

Solution: Adding a "wait an hour" block between initial trigger and next action (send the email) in the series solved our issue.

Why: In the milliseconds between when a user event came through and when their email address was available in intercom, the system that checks eligibility was registering these new users as missing email addresses. This is why some users got emails and others didnt - sometimes users got their data processed fast enough and sometimes they didn't.