Splunk in my box has admin and guest access. When I am logging in as a guest user, a few reports on the same dashboard are not visible. However all the reports in that particular dashboard are up and fine with admin user. Tried combination of index, but it's not working.
I think you need to check the permissions of the search. If the sharing is set to private or admin only, then a regular user will not be able to view the report on the dashboard.