Cause:

This may occur if:

Printing is done at low temperature or humidity
Paper dust on the guide board and transfer timing roller may stick to the paper and produce white spots.
Some types of paper, such as recycled paper, roughly cut paper, and high-friction coated paper produce a lot of paper dust, which will likely stick to the paper.

To adjust the following settings, pre-register the type of paper in use as a custom paper. For details about registering custom papers, see "Registering a Custom Paper", Paper Settings.