What I noticed is that since I had already overlapped the two segments that I wanted unified, that I could not separate the two segments. But selection for each segment, though visibly unified, were not quite unified as a connecting segment had smaller selection nodes than the real nodes. I shortened these nodes back to a real node and the segments became discrete (no visible connection). Since these two segments were colinear, I then connected one real node to the other and used the unify command and it now seems to work. Although it is still possible to select individual segments as well as the whole, newly unified line.