You might inadvertantly have put segments onto differing workplanes. Try selecting the segments that you want to join and format|place on workplane. Another thing that might occur is if the ends aren't coincident. Try modify|meet two lines with a couple of the segments, which also coughs up a warning if workplanes are different. For whatever reason, genie might have lost track of something. One or other of View|redraw or Tools|database|regenerate is supposed to refresh the drawing, which might also assist. Try drawing a polygon, exploding it, then see if the tool has magically restarted while the segments are still selected. I have seen this once or twice in 15.2, I don't remember how I sorted it, but I don't recall having to restart.