What kind of content, Operating System, and version of QZ-Tray?
I have the exact same printer in my office, so I'll be happy to test your code for you.
Printing blank labels is often caused by improper calibration. If you have the printer set for the wrong size paper, for example. Press and hold the feed button until it blinks twice to do a quick calibration to your paper size.
Sometimes they can be picky about the actual label stock, too. If you have black-mark labels, but the printer is set to web-sensing or gap, that can cause problems.