Each user profile can only receive a specific message once. If the user targeted has already received the message in question they will not be eligible again.
In order to receive Email, SMS, and Push Notifications, a user's profile must contain contact information. This contact information must be saved in the format documented here. When building a message, Mixpanel automatically removes users without the required information from the target list. Read more detailed information about specifically why push notifications might not be sending.
The following filters pictured below are automatically applied in addition to any you set manually:
Note that if the message is part of a Campaign, then date rate limits and prioritization can also affect sending.
Note that we have a separate help article for troubleshooting Push Notification issues here.