The client may have transferred to Abacus Admin, but an exception might've occurred because their NHS number or NI number was marked as invalid.
To check this:
Go to Abacus Admin.
Navigate to Service Users > Administration > Service User Import Exceptions Work Tray.
Review the list to see any clients with exceptions caused by invalid NHS or NI numbers.
If you update the NHS or NI number in Mosaic, the client should import successfully during the next import, and their exception will be cleared from the Work Tray.