Hi @EOhayon,
There are multiple potential reasons here. Could you confirm the following?
Are you using the marketing contacts functionality? If yes, are all recipients marked as marketing contacts? (HubSpot will not send emails to non-marketing contacts.)
Are you suppressing unengaged contacts in the Recipients tab of the email? If yes, you could uncheck the box. Otherwise HubSpot will exclude these contacts.
Are you using an exclusion / suppression list in the Recipients tab of the email? (Do not send to dropdown)
Do you have the GDPR settings enabled? If yes, you might have restricted email sends to contacts who have a legal basis for communication only. In that case, you would have to update your contact subscriptions.
Have some of the contacts previously bounced? You can check by filtering your recipient list by “Email hard bounce reason is known”.
Did you already hit your monthly email send limit? (5X marketing contact tier)
Have some of the contacts previously unsubscribed? You can check by filtering your recipient list by “Unsubscribed from all email is equal to True”
Best regards!