No expert here...
...but I hope this helps.
I never got much from cable crunches. Of course, I do 100 sit-ups every morning, so my abs are fairly strong to start with.
The secret is intensity.
First option....Crunch your abs as tight as you can and try slow movement. This might really make it burn as most cable machines (if your's are like the ones at my gym) only give you half the listed weight on the stack (50 is actually 25).
Second option (which I use)....get a dumbell and find a sit-up bench. Place the dumbell on your chest and do ab crunches. You use a dumbell because I find it easier to hold over the sternum with both hands, and you can go over 45 lbs with no problem. I currently do 110-125lbs. I reach muscle failure in 5-8 reps doing that.
If your abs are strong, you just need more resistance than most cable machines will offer, so you have to find a way to get more weight on the movement.