There are a variety of reasons why this could be happening but I've described a few of the more common reasons and resolutions below.  

1) Primary broadcaster is not authenticated.

NationBuilder uses the primary broadcaster for the primary website within a nation to send out notifications and password reset emails.  If you happen to be using a custom domain as your broadcaster email, you'll want to ensure that this is domain is set up properly within Settings > Domains otherwise the email will be unauthenticated and unable to properly send emails.  

Read more about why authenticating your domain is important here.

Instructions on how to configure a custom domain can be found here.

*Using a generic [email protected] email address does not require these steps as it is authenticated to send emails using NationBuilder by default.

2) Primary email is not being used.

If you happen to have multiple emails attributed to your profile, you'll want to ensure that the primary email (or email1) is the one being used to login and request password reset.