Printing in Duplex mode not work

I’m working on a project that needs to print double-sided. Retrive DEVMOD struct of the default printer and upgrade the property with dmDuplex DMDUP_VERTICAL.
LlSetPrinterInPrinterFile step with the library LL16 (free) my DEVMODE, but when I go to print does not work.

In the log I extracted the part which shows the printer settings before and after, but I have not detected any errors.

What’s wrong?