Hshark...
I'm curious what your ab workout consists of.
I started lifting again to combat problems with my lower lumbar (back) and found the most bennefit from doing forward crunches while sitting up in the chair thing using the whole stack (50 reps with 150#?) with a pulley system.
It seemed to isolate the pain and therefore heal on the front of my spine (closest to my belly button) but it also gave me "Keg abs"
I wish I had abs like my older brother who weighs about half what I do and did 500 situps every night when he was a teen (about 40 years ago)
I won't pretend to be an expert on lifting by any means, but I have definitely learned a lot since I began. As far as ab workouts go, a lot of people laugh at me when I do them because my ab routine looks a little bit goofy. The things that I do routinely are things that me and my buddy have either run across or have come up with. I'll give a list of things that I like to do on ab days that always work for me:
-incline situps
I usually do this as a warm-up set for my ab workout. I tend to do 20-25 situps on an incline ab bench
-incline bench with medicine ball
This is something that me and my buddy have come up with. Usually you see people doing situps and a partner throwing them a medicine ball in order to put tension on the person's abs. Our version of this is that you sit on the incline ab bench with the medicine ball, go half way up and hold so that your shoulder blades are off of the bench to about where your body is parallel with the ground. Then you proceed to throw the medicine ball straight up and catch it while holding your position. Every time that you catch the ball, you should feel it rip your abs, and it feels amazing. Really good for lower abs especially. I usually do this with a 14 pound medicine ball.
When doing this, the higher you throw the ball, the more it will pull on your abs. Also, if you toss the ball to where you are catching it in front of your face instead of down by your belly or at your chest, it will pull harder on your abs (which is good), especially on your lower abs. I usually do about 10-15 reps. The last thing I would say is to make sure you are tossing the ball straight up so that it lands in the center of your body and not off to the side. If you catch it to the side too much, you could develop a twist in your abs.
-Standing free weight swing
Once again, this is something that me and my buddy either came up with or ran across that I do just about every day. I would use either a 25 lb or 35 lb free weight like you would put on a barbell. Stand (usually in front of a mirror) with your feet slightly more than shoulder width apart, with knees bent so that you are almost leaning back. Then hold the weight out in front of you and move it straight up and down, while you concentrate on squeezing your abs. The motion should be kind of like you have a girl riding you, and you are "guiding her" up and down

(sorry for that, but it's the only way I know how to describe it without videos). Do 10-15 reps.
The next sequence of this exercise, you swing the weight diagonally at a 45 degree angle to the ground and swing it up above your shoulder. It should feel similar to as if you are chopping wood. Swing the weight back and forth, concentrating on catching the force of the weight with your obliques. Do about 10 reps per side. Much like the medicine ball toss, the harder that you swing the weight, the more it will rip on your abs. This exercise is all about balance.
-Flat bench ab workouts
For this workout, I simply sit on a flat bench sideways. You could also sit on the edge of a table. I begin by extending my legs and then bringing them into my body. Once again, this is about muscle concentration. Work on squeezing your abs as you pull your legs into your body. Do about 15-20 reps. Do not hold onto the bench while you do this, and do not let your legs hit the floor. Balancing helps work your abs.
Next, work your obliques by bringing your left knee to your right elbow, and vice versa. Do as many as you can do. The faster you do it, and the more you exaggerate the motion, the more it works your core.
For the next motion on the flat bench, you can allow your hands to hold you in place. Simply extend your legs parallel to the ground and just do scissor kicks as quickly as you can do them. Once again, concentration on ab muscles is what makes this workout effective.
-cable crunches
This one is really easy on your back. On the cable system, attach the rope handle if you have one. I usually set the weight at anywhere from 150-200 lbs. Grab the rope handles and go down to your knees. All you are going to do is squeeze your abs and do a crunch, pulling the weight down to the floor. It's a really simple exercise and depending on how much you concentrate your abs, it could be the best one of all of them.
-Ab ball flex
For this, simply grab an ab ball or yoga ball (preferrably a small to medium sized one) and sit on it with feet anchored against the wall. You will want to sit up pretty high on the ball and just squeeze your abs as hard as you can for a few seconds at a time. The harder you squeeze, the better it will work. Really simple and really easy.