If your barcode is not printing clearly, the source file wasn't created in the ideal quality. 

Try this sample label which was generated at the correct quality to test it: Sample Label

Please try reducing your print speed (try 3mm/second) and your print density (try 3 as well) to see if you obtain a better result. 

On PC: 

Go to Control Panel > Devices and Printers > Right click on Rollo > Printer Preferences > Settings Tab 

Adjust Speed and Density 

On Mac:

Access these settings directly from the print menu as shown below.