Place objects in floor plan

When I try to place objects in a floor plan they do not align correctly in the floor plan: Either it is right in elevation or it is right in plan but but never is it on the ground. What is causing objects to fly around?

It is a function of where the pickup point is when you move them.
If I remember correctly, the default pickup point is on the axis point.
If they are component you use regularly you can double click on the object to open it.
Select Tools from the SketchUp top menus then select Axes.
This will allow you to place the Axis where you want it to be.
Put it in the point you want to be the component.

Alternatively you can place the component and re-grab it from one of the inference points that is best for how you want to place it.

The other issue is the plane (surface you want to place it on.
If you move the cursor with the component on it to a point on the plane of the surface then move it to the location you want it it should be on the plane if an inference line to that point when placing the component.