The only thing I can think of: is it set to print all vs Page 1? This has "bit" more times than I really care to admit.
Does the duplex work when you are trying to print from something other than FileMaker?
If this printer works with Letter size paper, is FileMaker able to duplex that?
Solved this problem: Need to print all pages in record and it will print duplex. The back page is the second page on a 8.5 x 11 sheet of paper.