It says so right there in your Info Box (top left corner) - All Selected:2.
And your info tag says it has multiple elements (at the bottom).
It actually stands out like dogs balls when you know what to look for.
Sometimes if you have multiple elements invading the same space, they can cancel each other out.
You may have deleted both at once. Try it again with just one window.
Otherwise it could be a myriad of things: Filter Elements in 3D may have windows unchecked, the part may be corrupt in the 3D Window. etc etc.
Can you open up that window's settings and see it in the 3D preview (top right corner)?
What happens when you alt-click the window and insert a new one next to it, right there in the 3D window?
Cheers,
Link.