Subdivision modifier doesn't work on some edges I just started blender and I have a basic problem often when I use subsurf modifier. Most times it works well but also occasionally some edges wouldn't be smooth. For example, I was doing a tutorial this time making a faucet in the bathroom. I made this model simply extruding a circle. Then I added the modifier. But one round of edge is sharp as before like in the picture. It may be a simple mistake or misunderstanding but as I just started learning, I can not really figure it out :( It also often happens when I add more shapes into models. I appreciate your help so much!
Yeah, there are several double vertices. You can check by Alt-leftclicking the questionable edge, and move the now selected edge loop down.

To repair, select all vertices in edit mode, and click Mesh > Clean up > Remove Doubles.