Hi Chuck,
You can refer below link to setup external authentication with LDAP for FMC, FTD login, in below document, it also shows you can use packet capture tool to verify login process. I would recommend you to open a TAC case for further troubleshooting assistance.
https://www.cisco.com/c/en/us/support/docs/security/firepower-ngfw/215538-configure-firepower-management-center-an.html
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You can also learn more about Secure Firewall (formerly known as NGFW) through our live Ask the Experts (ATXs) session. Check out Cisco Network Security ATXs Resources [https://community.cisco.com/t5/security-knowledge-base/cisco-network-security-ask-the-experts-resources/ta-p/4416493] to view the latest schedule for upcoming sessions, as well as the useful references, e.g. online guides, FAQs.
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Thanks,