[Nothing Appears But Grey Shadow]

I had the same issue, seeing a shadow instead of the STL. The solutions above did not work for me. I ran the inspector tool in meshmixer but it found no problems. I then used the Make Solid tool under Edit in meshmixer. In the object browser, you'll see the original STL and the new object that has solid added to it.

 

I made the original STL invisible and exported the new solid STL and, Al-hamdulillah, it worked perfectly fine.

 

 

[Why can't I see the imported stl's on the build plate sometimes? They are manifold, normal are fine... just shows a shadow.]

I had the same issue, seeing a shadow instead of the STL. The solutions above did not work for me. I ran the inspector tool in meshmixer but it found no problems. I then used the Make Solid tool under Edit in meshmixer. In the object browser, you'll see the original STL and the new object that has solid added to it.

 

I made the original STL invisible and exported the new solid STL and, Al-hamdulillah, it worked perfectly fine.