The issue has been fixed; when I checked the log files I noticed below error
08-18-2018 18:12:32.508 +0300 WARN CacheManager - Last run failed to evict requested bytes. Performing eviction in urgent mode for path=/opt/splunk/var/lib/splunk/audit/db
This error was due to the minimum space in index (I am using lab environment), I changed "Pause indexing if free disk space (in MB) falls below" setting under server setting to match my free space... and everything work fine now
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