Make sure the cam is clean, lighting is good and not too reflective.. I also use google shopper and it usu works great... tho not all bar codes are in inventory, but that's with any of those kind of scanners.
I had the same problem initially, it really helps to hold the EVO with two hands. Once I did that shop savvy read the bar code. You might also want to try Google Shopper... it seems to be less forgiving while trying to scan a barcode.