Epson ET-7750 2 sided printing
Can't print two sided even though boxes are checked
MANY current and slightly older Printers now default to the AirPrint "driver", but include a full-featured driver, and in some cases 'Generic Postscript' Driver as options.
It is really easy to accidentally install the AirPrint driver, and the AirPrint driver does not support any printer special features such as automatic duplexing on request.
You may need to install a new Printer instance, and be exceptionally patient while your Mac looks around your network, chats with the Printer, and selects SEVERAL Drivers for you to choose from on a drop-down menu. The default is usually AirPrint, but another Driver may be available of you know to look for it.