Why do some people fail in good fortune-telling and bad fortune-telling come true?

It's just a psychological reaction. Everyone pays attention to the good, but is unwilling to accept the bad. Some conveniences are deliberately ignored. If you count all the hexagrams, there shouldn't be much difference between good and bad. Moreover, things that are sure are generally not counted, and they are not sure if they are counted. It is normal to have a higher probability of failure. Statistically speaking, your sampling is inherently biased, which is also a reason for the error.