This is usually for one of two reasons:
The contact is not in the right sector
The contact is already at the client but has been marked as Has left
To view existing contact on a client who has left:
Click Clients
Click the Contact tab
Click the Eye icon to view the contacts
This will list all the contacts including those who have left, to remove the Has Left flag:
Select the contact
Click the Details tab
Click within the Has Left tick box to enable editing
Click within the Has Left Tick box to remove the tick
Click the tick icon below the Has Left box to save the changes