Waitress refused the voucher

Today I went to circus circus Musgrave and when I had to settle the bill I asked the waitress by the name of Faith to use my voucher but she refused and said I can't use my voucher to pay for my meal, and since I didn't want to cause a scene I paid for it in cash. What puzzling me is that why would they tell me that my card was preloaded with a voucher that I can use anytime if I they gonna tell me that I can't use it. I wonder if their waitresses are taught customer service.