I imagine you have a setting in your printer driver that is making an "automatic" adjustment to a resolution that corresponds to something lower than 600 ppi. Try stepping through all of your settings in the various Print options, including the preferences for your specific printer. Double check references to size and resolution. If you see something that might affect the outcome, try changing it and testing. . That, for now, is about all I can suggest.
If you've done all these things already, sorry, just trying to be helpful!