The most likely reason is that the printer is connected to a new USB Port. Windows will register Label Printer as a "new" printer whenever the USB port changes. Thus when you print to the "old" printer, it simply keeps it in the print queue. 


This behavior exists so that you can run multiple Label Printers from the same computer, each with different label types and settings.


How to fix (for Windows):

Open your "Control Panel" -> go to "Printers and Devices" and note which Label Printer is "active".

It may be Label Printer, Label Printer (Copy 1), Label Printer (Copy 2), etc. 

Be sure to print to the same active printer name when printing.


Or you can unplug and re-plug Label Printer into the same USB port as before and that should restore your printing without further changes.