I have a Z3100 and the one and only problem I have had with it in the 7 years since I bought it was the printer driver (OK, I had to get the belt replaced, but that's normal after 6 years).
The first thing I would suggest is to print a test print from the printer control panel - if that's printing OK, then your printer is fine.
The second thing would be to access your printer over the network by entering its IP address in your browser. If that is OK then your network connection is OK.
Do the same thing with a USB connection to check that you have proper USB connectivity from the Printer Utility.
If that is all OK, then I suggest you uninstall and re-install the driver.
If that still doesn't work I suggest you try it from a different user (I don't know about the Mac, but on Windows you can have problems with the user profile that can mess things up).
If that still doesn't work, try it from a different PC ... or borrow a laptop if you don't have a PC available.
The reason I'm making these suggestions is that it sounds to me like your printer is loading the paper correctly.
I know a number of Z-series owners who've had recurring problems with the printer driver: a Windows update is enough to screw things up. On my own PC I couldn't access any of the color management features (like create new paper profile) and I also couldn't do it from two other PCs (one Windows 7, one Windows 8 ). I was able to do it from my laptop over USB (Windows 7), but not over ethernet. Then ... for some reason unknown to me ... I can now do everything from my PC over ethernet. I contacted HP several times about this problem and they just said that they had no knowledge of any problems of the kind, and it had to be a problem with my network or with my PC. They even escalated the problem to their development team (allegedly), who came back eventually and said that there was no problem with the printer driver.
I updated the printer to the latest firmware and installed the latest printer drivers, but neither made any difference ... so it seems to be an OS/network level issue that affects the driver.
Robert