Light
Face the light, do not shoot into it. Even, bright light (outdoors, or a well-lit space) helps the app see your body clearly. Harsh shadows and dim rooms are the usual reason a read comes back low-confidence.
Frame and background
Keep your whole body in frame for the entire motion — a common mistake is cutting off the club or the follow-through. A plain, uncluttered background helps too; a busy background gives the app more to get confused by.