Most phones/tablets today have a camera with autofocus. This makes scanning barcodes very quick.
The barcode scanning software relies on a sharp image to read the code. On phones without autofocus images are blurred when the camera is held close to the subject. You may try holding the camera further away from the barcode but this may mean the barcode is too distant for the barcode reader to interpret.
We recommend only using cameras with autofocus.