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.
Control panel
In the print settings, select the Advanced tab if there is an Advanced tab, and then select Best as the Quality.
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.