There could be a whole host of reasons why this happens but here's few points to check before you panic....

Starting with the barcode first:
- Is the barcode damaged ie. scratched so the code has bits missing, or is scrunched up?
- Does it have white space/quiet zone at either end of the code, so the reader knows where it starts and finishes?
- If you are using printed labels in colour, ie. not in black and white, check the colour scans.
- Is it a valid recognised barcode ie. produced using a barcode font or software program and registered in your system?  

Scanning Device
- Is it working, turned on and connected to the IT system/software?
- Check the reader isn't damaged, ie. cracked lens or dirty.
- Check the handheld is configured to read the barcode symbology.