Chien-Shiung Wu:

"Wu’s experimental discovery in 1957 confirmed a theory by Chen Ning Yang and Tsung-Dao Lee, who were then awarded the Nobel Prize in physics later that year, even mentioning Wu in their acceptance speech...Although she designed and undertook the experiment that confirmed the theory for which the Nobel was awarded, Wu was not included in the prize." (American Institute of Physics)

(Nobel Foundation Archive)

Some physicists argued that Wu should have been included in the prize for being the one to provide experimental evidence for Yang and Lee's theory of parity violation and believed that she was excluded from this award simply because she was a woman. 

1957 Nobel Prize Ceremony (The Chinese University of Hong Kong)

"Chien-Shiung believed that she was victim of industry-wide sexism. She was not the first female scientist to feel overlooked by the Nobel panel, nor was she the last. But Chien-Shiung did not allow this snub to prevent her from continuing her own research." (New York Historical Society)

Another theory others argued was that it wasn’t a matter of sexism but complications over the publication date for the experiment alongside other competing groups also working on parity violation. Furthermore, Wu and the other competing groups published their findings in early 1957, and according to the Nobel rules, the 1957 prize could not be given to work published in the same year. There was also a rule that only three winners could be named. However, starting in 1958, Wu was nominated for the Nobel at least seven times by Nobel laureates, including Willis Lamb, Polykarp Kusch, and Emilio Segré. Still, she was never awarded the prize despite the many views expressing that her contribution went beyond experimental prowess.