I believe that you need more lifting area on the fuse. Almost any plane will knife edge if going fast enough, but high speed is not really part of 3D flying.
Many years ago, a friend let me fly his new Sportster .40, and I did my first sustained knife edge. My friend was not a good builder, and the nose was kind of square with rounded corners, the covering was all white (dull!). I offered to refinish it: I resanded the nose round, made the plane really slick all around, and recovered it with a great yellow/red/metallic blue color scheme. Well, it never really knife edged again [&o] inspite of a hot Rossi .40 with tuned pipe pulling it (but it was much prettier and faster .
I picked up a few clues from that story.