Are you comparing like vs like vehicles? I.e. a buggy will naturally tend to fly more like a "line-drive" whereas a SC truck will just about always be a "pop-fly."

However, one way to induce a more "line-drive" flight path is to lift slightly on the bottom of the jump, squirt the throttle on the top and then use brake in-air to bring the nose down. Or, they may actually be going quick(er) on the run-up, coasting the take off and using a mid-air throttle squirt to bring the nose up. Will vary with the car type and jump construction/distance and is kinda hard to explain in text though - a difficult skill learned only though practice time, practice time and more practice time...