There are issues if you uploaded the images with an older version of Nikon Transfer. It does something to the NEFs that makes them unreadable for Lr.
Apparently if you upgrade the Nikon software, the issue disappears.
Other than than there is no reason, unless your Lr version is too old. Support for the D600 was only introduced in Lr 4.3. Anything older than that, and it's a "no go".