Confirm that you are printing on plain white copy paper.
If not, do so before seeking service to the printer. Even if the print media you are using worked in the past, please try this in order to eliminate your print media as partial cause of the issue. Specialty papers (like letterhead stock, textured paper, label stock, and card stock) can lead to print quality issues such as this.
For more information on recommended print media, see the chapter entitled 'Understanding paper and specialty media guidelines' in the printer's User's Guide.
If none of the tips above help resolve the issue, your printer may need service. Contact Lexmark Technical Support for other troubleshooting suggestions.