Clear/clean cache from apps that have ads. Then restart phone and check for updates for hosts, and restart again if updates are found. When that happened to me before that's how I fixed it. And to clear/clean the cache use something like Clean Master from the Play Store.
Just FYI, the latest version of Clean Master pretty much greenified all my system apps when I accidentally pressed the boost button. Had to restore to undo the damage. Recommend App Cache Cleaner instead.