Why are coupons not working?
1. You entered the coupon code incorrectly. Check again.
2. You do not meet the minimum cart value requirement.
3. You are trying to apply coupon on a book that is on sale. We do not allow using coupons for books that are on sale.
4. Perhaps the book or the diagram you are trying to purchase is not eligible for coupons (that usually doesn't happen).
5. You are trying to enter wrong combination of coupons.
So, check again and try again.