If one user has some clients missing from the client list check at the top of the screen what Client Type is selected.
Instead of Client Type being All Clients, it may have Current or Non-current selected.
Click the drop-down arrow and choose All Clients and the missing clients should appear.
Note: If you are using the SQL version, you will need to use the Find By: function to display your client. To see all clients, type * into the find field, then press the magnifying glass icon or press Enter.