Barcode Scanner not working correctly on Galaxy S II
After spending the whole week searching for this issue and testing different settings and tricks recommended by users from other forums, I have finally found the culprits.

It seems that the least mark of your fingertips on the lens of your phone's camera will make it less sensitive to QR codes and Barcodes in general.
That and also not having AutoFocus On.

Things I did that helped:


Clean the camera lens with a soft cloth to remove fingerprints or other marks
Turn on AutoFocus On your camera, if it's not On
Try setting camera Action to Macro, this seemed to make it recognize codes faster.


I hope these tips can help others as it helped me.

Edit:

Also check this Forum for more tips on this matter.