What size and ratio are the barcodes?
You must leave sufficient blank space on either side of the barcode to create a quiet zone. You must also select a good ratio/element size if the default values are not being used, and you must allow adequate room for the barcode to be printed on the label. If you ignore these requirements, the barcode that is printed may be unreadable.
If you need further assistance, please attach some images of the printed barcodes so I may see.
Do you have a barcode scanner to test if these are readable yourself?
Are you printing these from the Amazon website, or are they printed using different software?
Ok, you could try adjusting the Dots Per Inch to a lower setting in your print setup to allow you to print thinner, more easily scannable barcodes.
Another thing - what type of labels are you using? Are they listed as compatible with this thermal printer?