Two possibilities or a combination of both. You either have a wing or tail incidence problem or the engine thrust line is off. Too many modelers believe that an ARF can be assembled and flown right out of the box. I made this mistake twice with H9 products. The most recent was a H9 Super Cub. For those who are unaware it has built-in positive incidence on the horizontal stabilizer. This was designed for those who don't know how to take-off a tail dragger. It took me buy surprise when the tail popped up after releasing up elevator. In the air with the throttle set at 75% it took some down trim to compensate for the stab placement.