I'm having trouble joining paths in Illustrator.

History: I imported a plan from AutoCAD into Illustrator, selected & copied the lines that I wanted (contour lines), and copied & pasted them into the actual illustrator file that I am working on. Many of the lines are discontinuous, through no fault of Illustrators.

If I use the Selection tool on any contour path, all the contour paths are selected. If I use the Direct Select tool, I can select a single path or anchor as normal. Insofar as I can tell, I can manipulate, delete, and add anchor points as normal, and new lines to the contour lines.

What I cannot do is join two contour lines. I've cleaned up the paths, and selected the endpoints of the paths. Cntrl-J and the Join command all fail, however - I get the generic message about open paths & etc. I'm guessing that somehow the paths are linked, and this is preventing the join, but I can't figure out how. I've tried using the scissor tool on the endpoints, with no effect. I've also deleted the endpoint and used the new endpoint, to no effect.