Yes, it's the single-faced walls. You have some places where there are 3 planes sharing one edge. This creates a problem with the normals - which way is the inside and which way is the outside. You need to model those walls - the one where you just have a single plane - like you've modelled most of them as a U shape. There has to be one continuous inside wall where all the normals can and are facing the same way before that loop tool can calculate where it should cut the edge.

Yes, it's the single-faced walls. You have some places where there are 3 planes sharing one edge. The loop tool can't create a loop past these places. You need to model those walls - the ones where you just have a single plane - like you've modelled most of them, as a U shape.