It may caused by following reasons;

A. Label
The darkness of the label depends on the quality of the label. High-quality labels will produce darker labels
B. Print density and speed
In the preferences of the printer “NT-LP110A”, you can lower the “Density” and “Speed” to increase the resolution.However, it should be noted that excessively increasing the concentration will cause unnecessary black lines or the label becomes too hot and sticky, and reducing the concentration will make the label shallower.
C. Margin and page size
Please make sure that the document content occupies the entire label. If there is an extra border, the
label will be “reduced” and printed in a smaller size, causing the content to fade
D. Low-qualify source file
If it seems like your original low-quality print was designed for 8.5 x 11 size, please to check if you can change the label settings to be 4 x 6.
You may access these settings by going to your Control Panel -> Printers and Devices -> Right Click on ‘NT-LP110A’ -> Printer Preferences -> Page Setup -> Setting

For Mac:
Look for drop down menu under “Printer Features”.