delay on showing logs in fortiweb

hi everyone,

I have a fortiweb 1000D version 6.4.2. we set a splunk as syslog server on it and logs are available and real time without any problem  on splunk server. but if I browse logs on the fortiweb itself that logs are not Realtime and not showing the logs in past 1 hour. for example I can see fortiweb has sent some log belongs to 5 minutes ago to Splunk and can see that logs on splunk. but the same log is not showing in the log view of fortiweb. 

is there anyone who has the same problem?

thanks.