Quote from the movie Tin Cup: "No chance to hit it on the green."
Golf basic rules are easy; hit the ball, get it in the hole. But since there are only a few pro's and a lot of wannabees it might not be as simple as that. The basic rules for marketing and sales are just as simple; get your product to the clients. Again in real live this is not this simple.
There are a lot of aspect that influencing a marketing and sales campaign.
You have a good product, now how to get attention to this product, get this information to the right people, create awareness, etc.
The issue here is that all these aspects don't add up, they multiply. There were 1+1+0 is still 2, 1*1*0 always equals zero. i.e. If you have a good product, but cannot get this information to your potential clients, this will result in zero sales.
Using just a QR Code in these cases will not miraculously increase sales or even increase attention, the QR Code is just one aspect in the chain. For example, your QR Code contains a URL, but this site is not mobile friendly, the second aspect will still multiply your efforts in the QR Code with a value close to zero, rendering all the effort in the (Custom) QR Code a waist of time and money.
Get all the aspects that influence the success of your campaign up to speed, and your campaign might hit the green, or you might even get the hole in one.