Not upholding a discount offered.

I wanted to purchase two painting canvasses, from the PNA Centurion Lifestyle Centre, on Sunday 31 August 2025, 20% discount was offered on anything and everything, however, I had to pay the full price as the cashier told me that the price was already discounted, on the 28th of August I also purchased some painting canvasses and I was allowed 20% off the marked price, I asked the cashier why and I was told there are terms and conditions?