Cause:

This may occur if:

When printing in full color including the clear toner and the special color is set to high quality/printing in full color including the white toner
When printing in full color with the special color
Slippery transparent film is used

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.