In "The Rocking-Horse Winner," why didn't receiving $1,000 pounds on her birthday make the woman happy?

Because she only earned some hundreds of pounds per year.

Explanation:

The Rocking-Horse Winner is a short story written by English writer D. H. Lawrence, and it is about a boy that can predict the results of horse races under the proper conditions.

In the story, the woman received a thousand pounds for her birthday, but she found out that an artist could earn more money per year than her; according to the narrator, the woman wanted to be first in something but she could not even be the one who made more money in one year and that set her bad mood.