If it is a rough grass field, which it sounds like it is, with a forty size plane you will have to give it up elevator until you gain ground speed. As your speed increases, slack up on the elevator. You want to get the weight off the front end and onto the main wheels. Putting the plane up higher on the nose wheel won't do it by itself. By giving it full up with most of the pwer it gets the weight off the front and helps the ground control. Try different size wheels as well (bigger).
Good Luck