Check and see where the elevator is when the plane is trimmed for level flight. CG and incidence can cause you to put in elevator trim and when the weight is off the wings, this trim affects the knife edge pitch.

A nose heavy plane, for example, will usually carry a little nose up trim, causing the plane to tend to pitch toward the canopy in knife edge. A tail heavy plane would pitch toward the gear.

Wing and stab incidence is also a player as they can affect elevator trim. Changing the height of strip ailerons is an easy way to make a minor incidence change.