duplex print not working on hp6520 using libreoffice writer on linux mint 17.1

Please can you help solve the duplex problem?
The printer driver is hplip.
The software is up to date as per the Linux Mint 17.1 updater.
Driver test page prints duplex correctly.
Media is A4 autoduplex.

Please take a look to this threads:

Please try selecting PostScript (Level from driver) in the Properties → Device tab of your printer. The default value is “Automatic: PDF”, which can cause problems with several printers.

I was able to confirm this workd because I couldn’t duplex print on my HD OfficeJet 8600 from LibreOffice, but all other programs worked fined. Changing to PostScript solved the problem.