Separate data errors from image errors
If the generator rejects a value, check the format, permitted characters, length, and check digit. If a valid image will not scan, check whether the scanner supports and has enabled that symbology. A retail scanner may not accept Code 39 even when the bars are correct. A valid check digit does not imply the receiving software recognizes the product.
Inspect the blank space and bars
Quiet zones are part of the symbol. Nearby text, box borders, or a cut edge can prevent recognition. Avoid low contrast, inverted colors, gradients, and stretching. If a barcode is too wide for its label, choose a more compact supported format or a wider label rather than squeezing the image.
Check the actual print
Blurry bars, ink spread, thermal streaks, and fit-to-page scaling can make a screen-perfect code fail on paper. Export SVG or a physical-size PDF, print at 100%, clean the printer, and test a sample. For regulated or retail workflows, use a standards-compliant verifier; a successful phone scan alone is not certification.