Smart shopper voucher not working

I have been at Willow Crossing branch on the 17.2.2018. One of the items on my list was baby formula. The one I usually buy was out of stock. I had a voucher for Lactokid (15% off). It expired on the 18.2.2018. So I decided to buy that instead even though it was more expensive, but with the voucher it will be less. At the checkout the voucher did not work. It was not expired yet and no explanation was given. They said they will scan my card and voucher. Then what? it should have been sorted out right there and then! So I asked them to void it. I then had to buy formula that was for a younger baby. So not only are there no stock, but your vouchers does not work. I also returned peaches that were rotten before the expiry date and only returned them on the Saturday because I was not going to drive back because of their product being not to standard. Did I get funny looks at the store! I had to explain that yes now the expiry date is past, but it was not when I bought it! Must I drive back because of their mistake/ Do they not check the products on the shelf? I returned it when I went for my weekly shopping. They did not even ask me if I wanted a refund, just gave me another pack. Overall I find their fresh produce to be substandard, especially lettuce, I have to hunt for one that looks halfway decent. So what are going to be done about the voucher not working?