Loop cuts not a complete loop after rebuilding mesh

I’m trying to fix a mesh so I can bend it.
Why do sometimes loop cuts go all around a model

and other times, they don’t

I deleted the original triangular faces and then refilled the mesh (Alt-J didn’t work so had to do it manually). Each section was done the same way so not sure why this section isn’t happy. I’ve checked for double geom and I made sure all normals were outies. Dragging vertices, everything moves as expected so it’s def all joined.

My questions are:

  1. What causes this?
  2. How do I resolve it? (other than fixing it by joining vertices afterwards - that’s fine if I’m adding one, but if I want to add 10 it’s a different issue)


Only reasons it would not work is if there is a triangle, Ngon or hole along the loop to stop it from going all the way around. It will also count hidden geometry as holes if you have any.