Well, the message is already saying a lot about your problem.
You're running into the configured maximum limit for historical searches.
You can check the Monitoring Console to see how many searches are run, how many are being skipped, and which, and some other information.
There's a lot of possible reasons, and you can find a lot information when using Google on "Splunk maximum number of concurrent historical scheduled searches", including a few good blog posts on this very topic - check them out! 🙂
