Why a barcode will not scan (and how to fix it)

Validate the data first

EAN and UPC fail when the check digit is wrong. Code 39 fails when lowercase or unsupported symbols slip in. Fix the value before blaming the printer.

Then inspect the physical mark

Low contrast (gray on gray), missing quiet zones, under-inked thermal heads, and shrink-to-fit print are the usual culprits after data is correct.

Reprint at actual size with darker bars, more margin, and a quick scanner test on production media.