The limits of advertising
You can enable multiple ad networks for each format in the builder. Ad networks form a waterfall and the first working ad will be shown. For example, a pre-roll will only be shown once if the number of ads is limited to one in the plugin settings. This limit applies to the starting blocks, not the number of advertising impressions in one starting player.
You need to increase this number to show the pre-roll before each playlist file or if you have a loop playback. The limit is only valid for one session, i.e. it does not apply to other players and is re-installed after the page is reloaded.
Also some pre-rolls may consist of blocks of ads (VMAP) and in case the limit does not apply (you can disable VMAP support in the settings).
[Pro] also has additional options that allow you to limit ads in one ad block (in a row) from the beginning or from the second display.