My EF Edge needs down ele to hold inverted, and tucks to the belly in knife edge. I added another 1/2oz to the tail last week and it has improved, now balancing approx. 1/2" behind the spar and needin less ele to hold both.

I know this goes against the theory, but from the above I would try moving the balance forward slightly till the airplane flies neutral inverted, then try knife edge again.