After a heavy workout session my arms etc all feel the fatigue, but not the abs. During the workout I could feel it, to the point where I maxed them out and could do no more reps. I also tried adding resistance weight (those crunch machines in gym) but it seems I could only take so much without being forced to stress other body parts such as waist.
Then after like 20 minutes the abs just don't feel that soreness any more while the arms can feel it for easily a couple of days. Is this normal? I really want to improve my abs but it seems I'm stuck here.
Soreness generally only happens in the beginning due to lack of warm up, stretching, bad form, lack of endurance, or your body just has not adapted yet.
It is in no way an indicator of if you have worked out enough or not. Since a lot of exercises workout your abs anyways (they stabilize your body even when you are simply walking) they are already used to some stress and have adapted. For the first little while when you hit them really hard they may be sore the next day or two. But it won't last for long.