By default Exoclick In-Stream Video (VAST) has a frequency set on every 12 hours. It means that if you see the preroll on a video and refresh to see the video a second time, you won't see the advertising before 12 hours.
For your visitors it's nice but for you who are doing tests on your site, this may be considered a malfunction of the preroll off.
How to display VAST at each video play?
You just have to set the frequency on "Custom every 0 minutes":