This can occur in the following situations:
When more than one campaign is scheduled with the same scenario and recipients, during overlapping time periods.
When the filter entered for the campaign recipients results in an empty set of users. For example: simultaneous filters by group, functional area, hierarchical level, or smart groups. For these cases, there is the possibility under More options to choose the option: Assign to users who belong to any of the selected groupings.
When it is a campaign configured without sending notifications, it shows 0 sends only in the calendar.
When you have the following configuration combination: Allow users to proactively complete content (Settings > Awareness > Interactive Modules > End User View > "Show this section with all active Interactive Modules on the platform") and Do not assign already completed content (Settings > Awareness > Interactive Modules > Interactive Module Assignment: "Assign only to recipient users who have not previously completed the module in question") and the user has completed the content proactively or in another campaign and is then included in a scheduled campaign.
When it has not been sent due to an issue with your own mail server. An error message will be shown in the calendar for that campaign and also next to each user in the campaign details.
For the last case, if when entering each message you see a text referring to failed authentication, you should check the connection in the Settings > Email Server section. If the password for the account used has been changed, you will need to update it on the platform.
In some cases, it is necessary to set a limit on the maximum number of emails sent per minute. For example: for Office 365 accounts, the recommended value is 25, and it may be necessary to decrease it further, for example to 15, if unexpected blocks or delays in sending are observed.