In the very beginning of the story, she seems to be sad: "There were tears on Hazel’s cheeks, but she’d forgotten for the moment what they were about." So we see that even at the beginning she was not "awake" on any conscious level. Maybe subconsciously she was aware that Harrison had been taken, and that she was sad, but the impulse wasn't enough to bring into the forefront of her mind. However, she seems more likely to "wake up" than her husband; in fact, she suggests of the lead birdshot he wears that he "could just take a few out when you came home from work", to which George lectures her about breaking the rules.
However, after witnessing her own son shot down before her eyes, all she can say to George about her tears is that "Something real sad on television" had happened, but that she always forgets sad things. She and George go on with their mundane, repetitive conversation about George's handicaps. Hazel doesn't wake to the realization that her son was killed in the name of equality. And, the story, a dystopian social commentary, works better because of it. Vonneget wasn't trying to write a story about people defying the odds with success, rather how society shuts down those defiant people, and how Hazel was the perfect prototype of "normal".
hazel was not so much bright as george was, she was naturally handicapped and she could'nt remember the things easily.so at the end when she saw her son dying she was sad unconciously but could'nt remember why she was really sad.so till the end she was not able to remember things.