The following causes can be considered.
Solutions
Select the appropriate paper type setting for the type of paper loaded in the printer.
List of Paper Types
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.
Select the Maintenance > Print Head Alignment > Horizontal Alignment menu on the control panel to align the print head.