The Z1 is printing light or with poor quality. Not all the text or graphics are being printed on the disc.
Intensity. Increase the print intensity. Go to the File Menu - Choose preferences. Increase the intensity one or two notches darker. If quality does not improve, continue to increase intensity.
Media/Discs. Make sure you are using approved media. See this article for a list of compatible media.
Ribbon. Try a new ribbon. The ribbon may be defective.