A box reaches the dispatch bench, the operative scans its label, and the reader rejects it twice. Nothing appears wrong until the surface is examined closely. The print may have weak contrast, a crease may cross the symbol, or the encoded number may not match the order record.
Reliable identification depends on more than black marks on adhesive stock.
The barcode format, label dimensions, substrate, adhesive, print method and data all need to suit the job. Start by examining where the label will be applied, how often it will be scanned and what information the receiving system expects.