The following causes can be considered.
Solutions
Select the appropriate paper type setting for the type of paper loaded in the printer.
Solutions
When printing on plain paper, print using a higher quality setting.
Windows
Select High from Quality on the printer driver's Main tab.
Mac OS
Select Fine as Print Quality from the print dialog's Print Settings menu.
Solutions
Align the print head.
Windows
Click Print Head Alignment on the printer driver's Maintenance tab.
Mac OS
Select System Preferences from the Apple menu > Printers & Scanners (or Print & Scan, Print & Fax), and then select the printer. Click Options & Supplies > Utility > Open Printer Utility, and then click Print Head Alignment.