1. Incorrect power point: the strength of abdominal muscles and back muscles is not used, and the movement cannot be completed because it relies too much on the strength of waist muscles.

2. Abdominal muscle weakness: The abdominal muscle strength is not enough to support the body to maintain stability during sit-ups, thus affecting the completion of the action.

3. Incorrect posture: Doing sit-ups without correct posture may lead to uncoordinated movements and difficult completion.

4. Strength of the back muscles: The strength of the back muscles can't support the body to maintain stability during sit-ups, thus affecting the completion of the action.

5. Other physical reasons: diseases such as lumbar disc herniation and lumbar spondylolisthesis may also lead to the inability to complete sit-ups.

It is suggested that proper warm-up exercises should be carried out before sit-up training, and correct posture and strength methods should be paid attention to at the same time. If you still can't finish sit-ups, you can find a professional fitness coach or doctor for further examination and training.