Visibility Issues are among the most common issues with Revit. These can usually be chalked up to the following...
Category is turned off in the view
Object Graphics or Visibility overridden (ie., set as transparent, turned off)
Worksets (turned off in Visibility Overrides)
Object being blocked by another (like a floor slab)
Keep in mind that when working in a team environment (Worksharing) others may be editing your settings without you knowing. One way to mitigate this is by making some initialed views, especially when it comes to the overall 3D and Section Box views. Another common solution is to yell at the people who are doing this.