Quite simply, there is not enough exercise, and everyone's muscle endurance is different. Maybe because you have been training for a long time, your abdomen has become accustomed to this kind of energy consumption, so it will not be sour after training. But for the sake of effect, it is suggested to strengthen training and increase the number of times and groups of practice. If you need equipment, you should also increase the weight of the equipment. I wish you a perfect abdominal muscle as soon as possible.