I've had this problem forever, but never tried to correct it.

I usually receive ParaSolids of surgical instruments exported outof UG.
Importing the ParaSolid as a SW file is uneventful.

If the new SW file is a part, I can drag it into an assembly.
However, if the new SW file is an assembly, I can't drag it intoanother assembly.
It errors "You can not insert a document template into anotherdocument."

When I open the new SW assembly file, there is no file name at thetop of the tool bar until I click save.
After clicking save, I can drag the file into another assembly.

If I open an assembly that has imported assemblies, I get the errorabout "does not match internal ID".
I just tick "don't ask me again" and keep going.