STL file saved as a SW Part can't be moved when placed in assembly

I have some SW parts, which were originally STL files. That is,  I opened the STL file, and saved it as a SolidWorks Part file.

However, whenever i attempt to use those files in an assembly, those parts are immovable.

  • They are not fixed.
  • If I use the "Move Component" command, it will not permit me to select those parts.
  • If I attempt to use any mates, no part of those parts are selectable.

Is this a limitatin of SW, that you cannot move or mate to imported STL files?

Thanks for any advice.

-Joe

SolidworksAssemblies