$5.00 Off Coupon

I had two $5.00 off $25.00 coupons the clerk rang up two separate transitions for $25.00 each at the end of each transitions I asked if i had enough to use my coupon he said yes it came off. I checked my receipt after I left and saw that it did not come off. I went back into store talked with manager she said that subtotal had to be $25.00 to use coupons and there was nothing she could do about it . We Are on social security and I try to use coupons to save . she said she would train the guy on how to ring up coupons but I would have made sure I had enough to use them but he said it was enough