The checkbox next to the feed name should be ticked for the feed to be enabled and be automatically checked for new items.
When a feed is added, it will only pick up new items and not anything already in the RSS feed unless you press "Force Download".

  Name Feed URLs  
Feed1
https://nzbindex.nl/rss?minsize=25000&maxsize=&sort=sizedesc&max=10&groups=alt.binaries.games.encrypted

  Next scan at: 08:23 Checking interval (in minutes, at least 15). Not active when you use the Scheduler!