Morrisons review: saving stamps

I have used these for a number of years and never had a problem, I went in the store at catcliffe and spent roughly £70 gave over a full card and was told I could have no change back and would lose the rest of the stamps, I have never had this happen before I paid in cash and have since spent the stamps and wont be collecting them again, this also means I no longer go to morrisons every week like I used to do.