Applying Discount Code to a product

Hi,
I have created a discount for product variant, and applied for the same discount coupon for a product in the admin panel by clicking on Catalog --> Product --> Bulk edit product variants.

So in my web, i have selected the same product, to which the discount coupon is applied, and in the cart, when i tried to give the same Coupon Code, it is given me  a message like
"The coupon code you entered couldn't be applied to your order " . Can some one please help me how to apply a discount coupon for a product variant.

The below line is always returning null to me..

var discount = _discountService.GetDiscountByCouponCode(_workContext.CurrentCustomer.DiscountCouponCode);

This is in ShoppingCartController, PrepareShoppingCartModel Method.


I have checked where actually the code is setting 'DiscountCouponCode' property of customer class, no where am seeing it is setting the value, except when u apply the discount coupon post action.