It's likely caused by dirty heads or bad paper. Since ZINK is a thermal technology, it dpends on the thermal head being clean. This is also why the paper packs include a cleaning/calibration sheet.

Try a new pack of paper, which will rule out bad paper and dirty heads. Most ZINK printers dislike sudden paper changes (not a problem with dye sub), so use the current pack up to avoid sync problems. This may not be the case every time, but it's the norm with most ZINK printers. If this doesn't fix it, then the thermal head is bad.