Answer
Switching to the NiceLabel driver for that printer, with the 'Always use driver settings' option enabled, resolved this specific symptom — a USB-connected label printer accepting jobs into its local Windows queue but silently failing to actually print them is often a driver-configuration issue rather than a connectivity problem, and forcing driver settings to take precedence (rather than letting the application's own print settings override them) can be the fix for this class of USB label-printer problem.