Recently, I have had users that were not able to select Rooms or Spaces in their plan views. This is due to a new feature that was added to Revit several releases back.
To select a Room or Space in Revit, you need to roll the mouse around the Room or Space and find the X, and then select the X.
Several releases ago, they added some new select options in Revit. One of them was Select Element By Face. This is very useful when trying to select floors or ceilings. Prior to this select option, you had to select the edge of the floor or ceiling, which usually lines up with the wall and it was difficult to select. When the Select Element By Face option is checked, you simply pick a point in the middle of the floor or ceiling and it will select the floor or ceiling.
This added feature has a negative side effect. If the Select Element By Face is checked, you will not be able to select Rooms or Spaces. You can roll the mouse around all day and you will not see the X if there is a floor or ceiling turned on in the view. To select the Room or Space, you can either uncheck the Select Element By Face option or turn off the Floor or Ceiling.