Vodashop refund policy up to ****

I went to vodashop at NC Mall to put on a screen protector for my new phone. The lady consultant that assisted me never informed me that I must take a picture of my slip for return purposes. But she did say that I get 12 months guarantee. Now after 6 months I see that the protector is almost gliding to the right which is making it lift at the top right corner. I went to them now, they say if I font have the slip there is nothing they can do. I told them that I can provide my bank statement as proof, they dont care, if its not the slip, they wont help me.