It looks like the issue it the heat and print speed are too high for the orientation of the barcode. If there is anyway you could rotate the label layout by 90 degrees then you would get a lot better printing performance.
If you cant adjust the layout then you will need to reduce the print speed and the heat/darkness setting and increase the X-dimension (size) of the barcode.
The elements in the printhead heat up in order to burn the image on to the label but then need to switch off and cool down when they are not printing a line or part of a character. With tightly packed barcodes in the orientation yours are in the printhead elements do not have sufficient time in order to cool down before they have to print the next bar of the barcode. Increasing the time they have between bars and reducing the amount of heat (darkness) that is used in printing should give you some better results.
Peter is correct. If you can't rotate it, then you need to print slower, and don't set the darkness any higher than you need it to be or the bars will bleed together.