Unfair disrespected service

My mom has opened an account now 09 January 2023 took a bedset for 36 months according to the catalogue she is supposed to get the 1000 cash back but she didn't and when I went to the shop manager was not sure why she didn't I also called customer service and I was advised that she does not qualify for cash back as she is receiving grant but my worries NOthing has been stating about not for pensioners plz help my mom