10% discount on next purchase vouchers

I bought a megabox Pampers for R329 and received a 10% discount on next purchase. Then Monday I went back and bought Pampers, wipes, formula to the value of R1100, and handed in my 10% disc voucher. It was declined. The cashier called the supervisor and they both checked and saw that the voucher has not expired and tried it again. Again it declined. By now I was furious. The supervisor informed me that the call center she needed to phone closes at 17:30, but she will phone in the morning and get back to me. Yesterday she phones me to tell me that apparently the vouchers cannot be used in conjunction with another promotion, that is why it declined. I then informed her that was not on the voucher. So I emailed Clicks with a complaint, and the lady states the same thing. How can you now claim that t&c's apply if the t&c's is not on the voucher. Where should I have read it from. I think handing out these vouchers is just a cheap ploy from Clicks to get people to buy more things from there store with no intention of honoring their commitment.