Have you been working on your core strength?
Can you plank for 5 minutes or do 100 crunches but do not have well-defined abdominals?
Maybe you do crunches for the simple fact of reducing abdominal fat?
There is likely a simple reason why, and one that is quite challenging, defined abdominals comes down to body fat percentage. If you have a higher or even a reasonable or healthy amount of body fat, the definition may be covered even if you have strong musculature.
Therefore, reducing your body fat overall will enhance your chance of having this look via exercise and diet modification.
Have you noticed more males seem to have defined abs compared to females? This is because females have a naturally higher body fat percentage. Boobs are largely fatty tissue (and this is healthy).
Essential fat for males is a tiny 2-5% compared to 10-13% for females, with some research suggesting 11% is critical regarding healthy menstruation (differing for athletes). With averages sitting at 18-24% for males, compared to the female average of 24-31%. So, not only is it natural for women to have a higher body fat percentage but, it is healthy!
Other than body fat, the hypertrophy or muscle growth of the abdominal muscles is also important, and doing 100 crunches is unlikely to be enough variety or stimulus on the muscles.
When I say variety, it is important to acknowledge the abdominals have different ‘layers.’ Targeting the muscles can help increase muscle mass. To do this I would recommend completing oblique focused (such as side plank variations) with an upper abdominal and lower abdominal fatiguing exercise.
I say upper and lower “fatiguing exercise” as I do not have any evidence to say an upper abdominal (such as a crunch) compared to a lower abdominal (such as leg lowers) show different muscle activation. AceFitness has a great article on this, and look at the fibrous bands that define each of the different abs to make 6. Keep in mind – this is all one muscle still.
Get rid of your Stomach Fat
Lastly, I would like to address trying to lose belly fat by completing abdominal focussed exercises. I can see where some people get this idea of spot reduction from, “if I exercise this area, it will help this area” thought process. Fortunately, you can stop the routine hour-long of crunches. And go for a walk, cycle, swim, dance, or gym class instead of with likely better outcomes.
We cannot lose bum fat by doing squats or glute bridges. We also cannot lose stomach fat by doing abdominal focussed exercises. This will increase the size of the muscle, but if your body fat is too high – it is unlikely to help much more than increasing strength in one muscle group.