Emails will typically start getting “dropped” if a user has previously bounced, unsubscribed, or reported the email as spam. Usually a dropped message is a good thing, it’s the ESP respecting the wishes of that user and preventing them from receiving an unwanted email.
Sometimes a customer will accidentally (or unknowingly) put themselves on your ESP’s suppression list. Simply remove the user from your ESP’s suppression list to begin sending emails to that user again.
For more information on dropped emails refer to your ESP’s documentation: