This issue is caused by the constituent having a mail preference other than the mail type being sent.  In order to send to this constituent, we must either have the mail preference removed or a new mail preference can be added to the constituent filtering off of a category with the specific mail preference of either Mail or Email depending on the mailings he/she is missing from.   

Steps to add or edit a mail preference:

1.  Open the constituent’s record who is missing from the mailing. 
2.  Select Communication from the left-hand side and select the Preferences tab. 
3.  On the Preferences tab, notice that the mail preferences already has a Mail preference defined for Appeal and business unit that the mailing is for.  
4.  Either Add or Edit the mail preference to allow for the mailing type.