Is Audience true?
The Editor always shows your changes even though Audience isn't fulfilled. Preview however only shows your changes if Audience evaluates to TRUE. Make sure that Audience is true on the page where your changes are supposed to happen.
Are you viewing the correct variation?
It's very easy to view the wrong variation or even be viewing the Original. Make sure that you have selected the correct variation by checking what it says in the preview bar down to the left.
Do your changes appear late, or does the page flicker?
This is expected in preview. The preview itself has to load before your changes are applied, so the page may briefly flicker and changes can take a moment to appear. When the project is live, changes are applied much faster. Click About preview in the bottom right corner of the preview bar to read more, or see Preview projects.
Do you see the preview parameters in the URL?
If you are having a problem with getting the preview or editor on your website you can try to switch from # to ? in the parameters. Some websites do not allow parameters that start with one of these characters so changing it might be a solution. You will notice this either by the parameters being removed from the URL when you try to open a Preview or an Editor on your website or that you get an error message in the dev tools console complaining about the parameters.
First, try to manually change this in the URL and reload the page with the new parameters. Then if it works you can go to website settings and change the default parameter character for this website.
To do this click on My websites -> click the website -> Edit website. Then change to the desired parameter sign (# or ?).