Searches are skipped because there are more searches than there are resources available to run them. They also can be skipped if an instance of the same search is already running. Use the Monitoring Console to determine which is the case.
If a search is skipped because it's already running then either the search is inefficient and runs too long or it's scheduled to run too often. Review the SPL to make it perform better and/or change the interval to allow the first instance to complete before another starts.
If searches are skipped because of a lack of resources then you may have too many searches scheduled to run at the same time. Watch out for a large number of searches scheduled to run at the top of the hour and reschedule them to other times.
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