Coupon system not accurate on app

I've given checkers a chance to correct the and respond in fairness, this is a second resort. The coupons offered by checkers doesn't have expiry dates and says while stocks last but yet when redeeming them, they do not apply on your checkout because.... Wait for it... I never closed the app on my phone correctly therefore it never refreshed the coupons available, even though I said I did it in store while shopping. This means they do expire. I asked the lady who explained about the app that it's a technicality that my expectations were not managed and she responded by saying that these apps are of a technical nature. I work in IT and am very fami**** with retail software and if it's a maze for me as a consumer, then I can't imagine for everyone else. She then apologised and said she can't help me.