I'm trying to create coving/cornices and some skirting boards for a room I'm making. I've created the initial mesh by making a 2D profile and using the extrude and shear tools to follow the edge of the room. I've spent some time cleaning up duplicate vertices and edges, as well as deleting internal faces, but when I apply a subdivision modifier to the mesh, one corner of it appears as expected, but the other three average out and cut the corner like the images below.
At first I thought it might be because I need to add a loop cut to both sides of each corner, but this hasn't been done on that corner, yet it works fine.
I'm fairly new to Blender, so I'm bound to have done something wrong somewhere here. I'm pretty sure I've cleared up all the duplicate geometry, and I can't see any additional vertices in the working corner (EDIT: I've double checked this by vertex selecting the working corner and comparing the number with a not-working corner, they're the same). Any ideas?
Another edit: I think I've found the problem, but I have no clue how it happened.
In face select mode, I noticed what appeared to be edges that have the centre dot you see on faces.
Selecting these and deleting faces doesn't break the mesh, but I have no clue how that happened.
Additional note: Deleted those faces and attempted to apply the subsurf again, but the same problem occurs, so I'm still at a loss on this one.