Having Meener Dinkla's attention is certain to help, so give a lot of attention to what he says. I hope I can pay some of his help to me forward to you.
Seeing you are printing from Photoshop and have OSX and a z3200 as I do, I've run into similar issues after I got my Z3200 four years ago. It's a great machine no matter what the Epson or Canon fans say, and I have always found their customer support extraordinary (once you get the right telephone number).
If you are printing from Photoshop, my best guess is after selecting File>Print, click on Print Settings. (Eventually you have to understand what all the settings do in this series of menus to really control the printer from Photoshop). The settings change when you click on Layout and select the other options in that pop up menu. (When you do this, by the way, you are also changing some of the print settings in the underlying HP Printer Utilities application too.)
I'd try two things first. After selecting File>Print to bring up the print menu, click on Paper Size to pull up your particular paper. Most common papers are listed here. But I don't see 24x36" on my screen so I suspect this might be your problem. If you see the same thing, you will need to click on Manage Custom Sizes on the bottom and follow the series of menus to create that paper size in the fairly self explanatory menus there. Make sure to understand what you set as print margins. It will thereafter appear at the bottom of the previous pop up menu, and that paper size needs to be selected whenever you change papers to that 24x36"paper size.
Before printing, do this second step.. Go to File>Print, click on Layout and select Margins. Make sure you have not selected "Clip Contents by Margins" (I usually select Oversize), or if you do, make sure you haven't clipped your image by your margin settings in the step above.
Hope this works. Photoshop actually interacts with the printer driver settings so you shouldn't need to go there first. You can check and change those settings in the HP Printer Utility menu later if this doesn't solve your problem.