Best situation is that you put it in a well lit spot and that you yourself see a clear barcode on the screen. If you try to do it w/ dim lighting- it isn't gonna work.
I've tried in different parts of the home and found that it had to be nicely lit, otherwise if it's dark, it won't scan
When it starts scanning, you actually notice the camera "focusing" on the image, if you have a clear image - let's say you google a barcode with a well lit monitor on a laptop or computer- it should work
Try using this one:
http://teachers.egfi-k12.org/wp-content/uploads/2009/09/barcode.gif
It was immediate for me
Chances are you're in a store when this happens and hopefully this isn't the sketchy store that's dark and all. Otherwise I don't see why you would pull this out in a bar or somewhere dark- this would make the worse pick up line in history "Hey, I can scan that for you"