This has been seen when using Managed Preferences on the Mac's.
If a machine had managed prefs, it didn't work, if they did not have managed prefs, it worked.
In one case it has been narrowed down to a setting seen in the attached screen shot. This is under Login Items and an option called "User may add and remove additional items". If the box is unchecked, printing with Pharos Popups will not print and if it is checked the client is able to print.