When filling a gap with a new face edges do not seem to subdivide correctly I'm learning Blender and following Blender Guru's Intermediate Modeling Tutorial to make an anvil. I made a small mistake and had a duplicate vertex that could not be cleared by the Remove Double Vertices tool for whatever reason, so I went in and deleted it myself then restored the face. However, the new face now has sharp edges that won't subdivide and I don't understand why.

I've attached a screenshot that will hopefully provide a little more context. I'm referring to the two faces on the tip of the horn of the anvil, relatively centered in the viewport. The edges are sharp and I'm trying to round it to a point. You may also see a similar issue happening at the corners of the higher part of the body towards the top right of the viewport.

P.S. I'm aware that I'm using the 2.8 Beta and he's using an older version in the tutorial, so I'm not sure if something would have changed between these versions that either causes this issue or the way to solve it just isn't as clear.
The most likely explanation is that the new faces normals are flipped.

In the image below, i created a cube, added a subdivision surface, and then flipped the normals on the top face. As you can see, the top face becomes sharp, but it is hard to see why. However, if you select it and do the "flip normals" command, it returns to normal.



This may not be your issue, but try selecting the affected faces and using either Flip Normals or Make Normals Consistent, and it will most likely fix your issue.