Most of the time it is due to excess fat deposits. However, if you are thin and have a protruding belly, it is often caused by lack of tone and release of the ab muscles. For this you would need to target your training on the ab wall with specific exercises aiming at reestablishing your postural balance.
Last but not least there is ptosis, an inferior displacement of an organ.
Ptosis in combination with a lack of toned abdominal muscles the ab walls cannot retain viscera and the belly collapses and creates a pocket where the intestines then rest.
So to make all this babble make sence… if cardio and nutition aren’t doing it, then stop with those belly bustin’ exercises and concentrate on core. Once the ab walls are strong enough, go back to your crunches and see the difference.
Hope this helps!