This issue occurs because there is an incorrect view range in the main view.
Revit site plan showing floor plan.
Under floor plan views select the level on which to base the site view.
About plan views in a 2d view display a floor plan a reflected ceiling plan or structural plan.
A floor or slab will be visible in a plan view if it is partially above the view depth or if it is within 4 feet approximately 1 21 meters of the bottom clip.
About the view range the view range is a set of horizontal planes that control the visibility and display of objects in a plan view.
You can find this tool in view tab create panel plan views drop down plan region.
Click view tab create panel plan views drop down floor plan.
Create a site view as follows.
In the new plan dialog clear do not duplicate existing views.
Draw the sketch as necessary then click finish.
Under floor plan views select the level on which to base the site view.
If the view range intersects the geometry of a level in your main view that level cannot be used as an underlay in the main view.
About plan regions to define multiple cut planes within a plan view use plan regions.
In the new plan dialog clear do not duplicate existing views.
In the project browser rename the newly created floor plan view to site.
In the main floor level the bottom of the primary range is set to main floor with an offset of 1.
In this video i show you everything you need to.
Get this revit file here.
Click view tab create panel plan views drop down floor plan.
You will need to define the plan region using sketch tool.