This may be for several reasons:
AWS Wickr Only:
Your admin may have turned off Out-of-Network Federation, in which case, contact your administrator.
AWS Wickr and Wickr Me:
The person you are looking for may not be a Wickr Member, in which case you have the option to send them an invitation to download the free version, Wickr Me, and start communicating securely with you.
You also may need to look in your "Blocked Contacts" to find someone who was blocked and no longer is listed in the Contacts list.