Damage to the barcode label
If a specific item will not scan, but other items with the same barcode will then it is possible that the barcode on that item is damaged. In this case simply type in the barcode or search for the product.
Leading Zeros
If a barcode number has leading zeros (0012345) then some older barcode readers may struggle to read these numbers. This will make the barcode unusable by that scanner.
Not Scanning at Point of Sale
If an item wont scan at Point of Sale then a good place to start is by looking at the item’s stock card. From the Point of Sale main screen click Window → Stock Cards or press <Alt+S>
In the Options Tab ensure that POS Lookup is selected.