AATL Member not being recognized

This appears different than the other posts I have seen - when I open the file containing the digital signature for this client, I am getting the warning that "Signer's identity is unknown because it has not been included in your list of trusted certificates and none of its parent certificates are trusted certificates."  When I click the signature properties, it says it has been issued by IGC CA 1 IdenTrust Global Common, which is shown as an active and approved member of the AATL.  What is the issue?