Now why didn't this work?

I was trying to hollow out this engine cowling for this plane using FSDS. I spaced out the outside cross-sections, then attempted to double back upon it on the inside to create a open space for the engine. However, no matter how far back I go, I only get the small indentation on the front of the fuselage. I don't know if this is the proper way of doing this, but it should work, so why not?

