Reason 1: The source file wasn't created in the ideal quality:
Solution: Print the sample label to test if your printer is working well.
You can download the sample label from USB Driver:
OR website
Reason 2: High print speed or density.
Solution: Try reducing your print speed (try 3 in/sec) and density (try 3 as well) to see if you get a better result.
Here's video tutorial.