DOUBLE SIDED PRINTER

I have three apple compatible HP printers (HP Laserjet 3700dtn, HP OfficeJet pro 8725 and HP LaserjetPro MFP M479fdw) all of them with duplexers and work great... today I tried to print double sided on all of them on a an MS-WORD document that I printed that way many times before... and the duplex option has disappeared from all the print drop-down window??? I tried everything possible to get it back but it is just not there? Can anyone help...???