Why Imac can't find printers on my network?

I have two Imacs. One is running OS10.6, the other OS10.10. I have no problem connection to the Internet with either. I have an HP printer wifi connected to my router and a Samsung printer Ethernet connected to the router. The IMAC running OS10.6 has no problem finding the printers. The IMAC running OS10.10 cannot find them. In System prefs> printers and Scanners, if I attempt to add a printer, none are shown in the IMAC running OS 10.10, but the IMAC running OS10.6 finds them readily. Running network diagnostics or Assist does not help. And it matters not if the IMAC running OS10.10 is connected to my network via wifi or Ethernet. Obviously there is something preventing the IMAC from finding the printers. What? Thanks for your help. merph