The lack of a six-pack despite constant training can have various reasons. Overtraining, for example. You are challenging your abdominal muscles too intensively and thus depriving them of the opportunity to grow during rest breaks.
Another reason can be one-sided training. If you focus solely on sit-ups, for example, the muscles only receive this one stimulus. For constant growth, however, they need different stimuli. You should therefore diversify your abdominal muscle training. Also remember to train your lower, lateral or oblique abdominal muscles with appropriate exercises.
Nutrition also plays a key role. After all, a six-pack only becomes visible at a certain body fat percentage (the exact value varies from person to person). A healthy diet tailored to athletes is the perfect complement to abdominal exercises if you are aiming for a six-pack.