If the plane is flying with a tail-low attitude, but is not overly-sensitive on the controls, then it's likely the incidence of the wing or stabilizer is off. Most likely, the wing is at a negative angle. This will cause the tail to go low in order to put positive angle of attack on the wing when the model's in flight and flying at a constant altitude. If the stabilizer is off a bit, you'd have to carry elevator to keep the model level, but the tail will still seem to be in the normal position.

Also check how the ailerons are rigged. If they're not exactly centered, those big ailerons can change the level-flight attitude of the model. You may find that they need to be moved a bit, and the model will then fly with the proper attitude.

Careful checking will tell you what's up.

(edit to add the aileron note)