There are different factors that trigger the Satisfaction Survey.
The most important is that the survey is triggered by the change of the status to Closed. Another factor is that the creation date of the post must not be older than the moment you enabled this feature.
Lastly, there is one more thing to take into consideration. Setting up your Satisfaction Survey to be sent in 50% of cases does not mean that in every 10 cases, 5 will trigger the survey, for example. But that every case has 50% of chances to trigger the survey.
Find out more about Satisfaction Surveys here!