Thank you for your post.
There are a number of areas why this may be failing. Without going through an entire list of things to try, I think it would be best if you look over the White Paper for External Authentication available through Knowledge Base Article #6822 at:
Also, page 62 of the FileMaker Server 12 Help manual at:
Let me know if this still fails, and what steps you have tried so there is no duplication of effort.
I was finally able to get this to work. Turns out I had to unjoin and rejoin it to our Domain as the handshake between the server and the domain had gone bad.