Well, as much as I can see, you always use the very first bolt
select $sel_faces[0] "Bolt_01";
And every time a new Bolt is imported it gets a new name. So what happens is: The new Bolt is imported and placed in world center, then the very first "Bolt_01" is snapped to the selected face. I'm not sure, but I think the import command offers a way to get the imported node names which can be used to get the correct name of the imported bolt.