RoboKiller does a great job differentiating unwanted calls from calls that you want to receive. Occasionally one may slip through. Don't worry, there are things that you can do to help improve the service not only for yourself but also for the entire RoboKiller community.
Open the RoboKiller app and look for the blocked caller entry in the Recents tab. Typically, it will be labeled as Blocked Call or Blocked & Recorded Call. Tap the "Hamburger menu" or three vertical dots to the right and tap the Allow button for the entry. This will add that caller to your personal "Allow" list so that they are not blocked in the future. Also, you can proactively add any of your own numbers to the Allow list to ensure they will never be blocked and will always ring through.