Bad service at PEP

Today I went to Pep stores in Secunda. I wanted to exchange a pair of tekkies I bought for my boy. I do not know what I did with the till slip, now they do not whant to exchange it for me. And I did exchange goods before with out a slip. They where very rude as well. I know that if you do not have a slip, they can not refund you. But I just wanted to exchange for a bigger size.