I've made several of the same poly line "join"s (weld) and now it wont do it.  I've joined a couple of the segments together, DS detects them but won't join them.... I did the same thing with proper results about a dozen times just before this . 

ETA:  I deleted all the related items in the drawing and started over and was able to get it to work properly...strange.

(this was after shutting down and restarting DS)