Some models bugging during simulation

[programming2C78B]

edit: It was because my model Opacity was down on this, and the stock had a hole the size of the inserts. Still have questions though. I find it weird how it resorted to only showing one face of the insert instead of fading everything? 

there are these inserts in my part, as 3x separate components. They can't be bodies because it wont allow me to use 3 different rigid joints on them.

they are all selected in my Setup.

 

When I simulate, they not only move but are also only a single surface and not a solid! Here is the hole from the top

and from the underside!

Here is it with opacity down

 and back to full. How come there is no "machining" done to these inserts?