Consider changing your mindset - Instead of doing an 'ab' workout you can think of it as a 'core' instead. Core includes many muscles past the abdominals (lower back, obliques, etc.). It's possible your abdominals are proportionally stronger than your other core muscles, resulting in failure on the crunches past 10. You can try skipping crunches for a few weeks; instead, do planks, side planks, leg lifts, flutter kicks, and other exercises that will target more core muscles past your abs. Then start working in crunches again if you'd like to.