I'd be interested to know that too.... Could just be the extra weight made takeoffs longer.

I remember the article (I think it was RCM) on Bob Fiorenze's F-18. In the article, he mentioned that for each wing, he had one servo deploying both flaps and leading edge flaps at the same time. So, in Bob's case, it couild simply be that for a relatively-heavy DF model with full or nearly full flaps deployed, the interaction wasn't right.

Interesting to note that once the leading edge flaps are deployed (alone or with flaps) these fighters' elevator goes to neutral, or in some cases, even to the "down" position. The leading edge flaps seem to move the lift vectors forward, and pitch the plane up. (the desired effect)

Here's a photo of an F-18 doing a sharp pullup. The tailerons are actually deflecting in the "down" position.