This happens most commonly for me when the bottom and depth values of a view's view range are set to the default of zero, and I've placed structural walls, which default to a depth condition instead of a height condition. In other words, placing such walls without first modifying the depth/height condition results in those walls being placed from the associated level DOWN below the view range.
You can easily check for this in your 3D view by clicking on one of the offending walls and checking its constraint values in the properties pallet, or in your plan view by setting the depth value of the view range down below zero and seeing if the offending walls then show up.
If this IS the case, select all of the offending walls in your 3D view, and change their constraint values so that they will exist with a height above the associated level instead of a depth below the associated level.
Other possibilities may include:
Walls placed with ANY constraint values that make them outside of plan view's view range.
Walls category turned off in plan view.
Walls placed in workset that plan view doesn't show.
Walls placed in phase that plan view doesn't show.
Walls placed in design option that plan view doesn't show.
