Start adding weight or band resistance to your ab exercises. Increase the difficulty of the lifts the same way you would with any other body part to give it new stimulation. You wouldn't expect your legs to grow if all you did was bodyweight squats, so you can't expect your abs to grow from doing a few crunches.
You'll progress by changing up your routine and making it more challenging. Try using bands because you have more options for the level of resistance and it's easier to hit different angles. My two favorite exercises are:
The bands can be around your feet or knees.
Again, the bands can be around your feet or knees.
If you're doing your right leg, bring your leg up towards the left side of your body to target the obliques (and the opposite for the left side).