In "The Story of an Hour" why do you think Mrs. Mallard has difficulty, at first, recognizing the realization that she is free?
Her reaction of a feeling of total freedom and happiness is not the natural reaction to the news of someone's death. Louise Mallard reacted, initially, with bitter tears, a more traditional response to the news that one's spouse has died. It is such a shocking piece of news--she didn't have time to ponder it beforehand, it's not like she knew it was coming--that the revelation of happiness in being free took some time working its way up through the shock.
Also, as a dutiful wife, she was supposed to feel sad; as a dutiful wife, in her marriage, she was supposed to be happy. It was inappropriate to feel discontented in marriage; during this time period, that is what women were made to do, to get married and be good and happy housewives. Chopin herself indicated that Louise might have completely repressed any feelings of discontent in her marriage--in describing Louise's face, Chopin writes that it showed a "repressed" tendency. So, Louise had become quite good and repressing, or trying to ignore or make go away, any feelings that she had in her marriage that weren't happy or content. Once you get into the habit of repressing feelings, it is a natural thing, and sometimes it takes a while for them to resurface.
So, a combination of practiced emotional repression of discontent during their marriage, the inappropriateness of the feeling of joy at her husband's death, and the unexpected shock of the news, probably kept her joy from surfacing immediately. However, it did surface, and given the time-frame of the story, only one hour, it didn't really take that long. She heard the news, she wept, she went to her room, she sat for under 40 minutes or so, and felt the joy during the course of that time.