You can set the probability of how a particular question in your survey will be shown to a respondent. This means that when you create a question, you can set the probability between 0-100%. If you, for example, select probability of 50%, there is a 50% chance of that particular question being shown to a given respondent. In short, 50% of respondents will see the question, and 50% will not. 

Note: if you find that the survey skips some or all questions then most likely the "% of respondents will see this question" is set to zero % or very low. Simply increase the % to show the survey to more respondents.

This is useful if you have a large amount of questions and need a representative sample for each one, instead of showing a long survey, you can get a sample of responses on certain questions.

To do this in the survey creator, simply drag the probability slider as you like. Note that the probability can be changed for each question (with a default probability of 100%).