This happened few weeks ago with one of our users. I noticed that the group which the user belonged to had "Data Source" -> "Connect" permission set to "inherited" and when I looked up the permission for that individual user for that data source, it said "denied" for access. Then I fixed the problem by giving the group Data Source Connect permission of "allow" and pushing it to all workbooks by clicking "Assign Permissions to Contents".
It happened again today and I noticed the group's data source connect permission is set to "inherited" again. I solved the problem the same way as before but why would the permission change automatically by itself?
Is there a log that I can look into to see when and how the permission changed?
See this thread for Permissions priority help. Also review the Admin manual: http://onlinehelp.tableausoftware.com/current/server/en-us/help.htm#permissions.htm
It could be there's a permission else where that is trumphing the base permission.