Please "check product eligibility" of the offer by clicking on the piggybank icon on the bottom of the app. That way, you will make sure you get the right product before you actually purchase it.
Depending on the barcode position in the product, you may have to try a couple more times for it to go through :) Please make sure you have the latest version of the app on your device!
Just give it a couple more goes at scanning, since sometimes this fixes the issue. You can also try restarting the app or log out and back in.