There could be a number of reasons for this.

1. The contacts are subscribed as Text but you are sending a HTML email campaign. Check that the email type you are sending matches the contacts preference.

2. A spam filter is picking up the email and blocking it from getting to their inbox. Although the preview may have come through, your email campaign may have had link tracking or open tracking on, which some picky spam filters will use to detect a possible spam email and block it. If unsure, test your next campaign by turning off link and / or open tracking.

3. The email being sent is being bounced back for a particular reason. Check your bounce account for any bounce emails and see if any match the particular email address that is not getting delivered to. Check to see if there is any specific message or suggestion on how to fix deliverability.

You can also ask a systems administrator for the receiving domain name if they have received the email or if they can search their mail logs to see why the email wasn't delivered.