I was having this problem this week and after trouble shooting, this is what worked:
Open ArcCatalog --> navigate to the folder the file geodatabase is located, right click --> properties --> under security tab there is a list of users, select the one(s) which you use, below it will tell you what permissions that user has. If your user doesn't have a check box that indicates they are allow to do everything, this may be your problem --> edit, (select user and check all 'allow' boxes), ok, ok, close and reopen.
Running as administrator worked as well but it only worked in that mode, I wanted to be able to edit the geodatabase when I was in a map.