It seems AddToCart function is not adding same product to the cart with different Attributes selection.
First time a product with size:8, color:black added to cart.
Second time same production with size:9, color:white can't add to the cart. There is no error message as well.
Below function also does not return any CartWarnings. I checked in debug mode and the attributes are different for the same product.
List<string> addToCartWarnings = this.ShoppingCartService.AddToCart(
ShoppingCartTypeEnum.ShoppingCart,
pv.ProductVariantId,
attributes,
customerEnteredPriceConverted,
quantity);
FOUND SOLUTION: GlobalSettings.Products had "Maximum ShoppingCart Item" = 1. I changed it to desired number and works fine.