Overcharged and no refund even after going back to store with slip!

Bought Dulux paint during the promotion 1/6/21 to September and was charged the wrong price. I went to Builders Rivonia and they couldn't find someone from the paint section then asked if they could take a copy of my till slip and they would phone me shoertly. Well needless to say no phone call and why I could not of been refunded the difference in store is still an absolute mistery, not the first time the price on the item is different when it gets scanned at the till!!!!!