Why does a barcode that looks fine fail to scan?
Because a scanner is far less forgiving than a human eye about the exact width of the gaps between bars. Our own testing found that barcodes fail because those white gaps close up, not because the black bars visibly degrade, so a label can look sharp and printed cleanly while still being unreadable to a scanner. See the full method on the barcode generator page.
What’s the single most common cause?
Print quality set too high, or a print dialog silently shrinking the page. Counterintuitively, a darker or “best quality” print setting lays down more toner or ink, which spreads slightly and closes the gaps between bars faster: the opposite of what most people reach for when a label looks like it needs to be clearer. Separately, many print dialogs quietly scale pages down a few percent to fit the printer’s non-printable margin; on a barcode already near its size limit, that’s often enough to push it over the edge.
What should you check before reprinting?
Print a line of known length and measure it, then try a lighter or larger print rather than a darker one. On desktop, choose “Actual size” or “100%” rather than “Fit to page.” If you’re printing from a phone, check whether the print dialog offers a scale control at all: some don’t, and shrink the page regardless.