My first thought was that maybe you were already in good shape in your abs, but, really, any yoga exercise will work you to your limit and you should notice it, no matter what your level of fitness.
It's possible you're not doing them deeply enough.
Or, if you do them with food in your stomach, you're not going to get the same depth of contraction as you would if your stomach were empty. So, if you're not doing this already, try it on an empty stomach, like first thing in the morning. My yoga students and I havce found that you can almost double the number and greatly increase the intensity of them that way.
Another way to practice getting them deeper, so that you'll feel it, is with the Mega Lift. This is one big abdominal lift which you hold while you dont' breathe. As long as you're holding it, you try to tuck your abs higher and higher into your chest cavity. Keep doing that until you need air. Then drop, gasp and wait until everything has relaxed before doing it again.
I describe the mega lift in detail here on my website:
www.audioyoga.com/AbdominalLifts2.htm
These will increase the strength of the regular ab lifts and build your abs even more quickly.
PS: I promised the people on my Yoga Snacks thread I wasn't telling you folks the secret of the empty stomach.
(They needed an edge somehow!) Now I'm sure they're going to wrap me in a pretzel position and put a stamp on my butt and squish me into a mail box.