You have a big Ngon that is not letting the cut go through. It’s a big topic so I suggest you first watch these two courses:
Blender Mesh Modeling Bootcamp:
https://cgcookie.com/courses/blender-mesh-modeling-bootcamp
Fundamentals of 3D Mesh Modeling:
https://cgcookie.com/courses/fundamentals-of-3d-mesh-modeling-in-blender
