I have beeen having a similar problem. Some gmail accounts receive the reset email and others do not. Non gmail accounts don't seem to have any problem receiving the password reset email.

I have now discovered that those that do receive the email have their email address as their username. Those that do not receive the email have a username of something other than their email address.

I wonder if others have a similar experiance?

For anyone experiencing this issue, I'd suggest looking into your SPF and DMARC settings.  Be sure that this all exists and that the values are consistent and correct.  

I'd also suggest that you avoid using any SMTP relay that connects directly to generic providers like Gmail & Outlook.  This is because when issues occur, you have no way to know what it is and how to fix it. 

If you use an actual dedicated SMTP relay provider (e.g., SendGrid, Mailgun, Postmark, etc.), you'll have tools to not only securely send emails, but you'll have a ton of other tools to further secure the connection/communication and reports/tools to troubleshoot any issues that arise.  

As an example, whenever this happens, I can look into SendGrid and search for the email and/or recipient to determine what happened.  I'll know if the email even made it that far, and if it did, what the result was.  

I've a couple of clients that recently received a Google's notification about Gmail and they suggested to make sure my client's domain has SPF, DKIM and DMARC records to avoid email reception problems; I guess you could make sure your domain has these three records configured correctly.