This may be for several reasons:
Your admin may have turned off In-Network Federation, in which case, contact your administrator.
Pro and Me:
If you originally found the contact using Contact Finder, you may need to turn Contact Finder on again. Contacts found with Contact Finder will only be able to be seen while Contact Finder is active. Other contacts will not be affected.
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.