i think you should try chaging the position at which you do your excersice. and try and find a position that will concentrate on all targeted areas.
References :

try doing leg lifts, lay on your back arms beside you or under your butt and lift yor legs six inches off the ground and hold then lower but dont touch the ground with your feet, this does wonders for your lower abs. and also try placing a small ball or pillow under the butt and doing this so you get more range for your legs…trust me this works you should see results quickly…….

try holding your legs extended 6 inches above the ground for about 30 seconds, then lower slowly hold again 30 seconds, do this as manytimes as you want if you feel you cant do it anymore do it 3 more times for maximum results. you should begin to feel the burn in your lower abdomin, it should work if your doing it right. but time is all your choice, hope this helps
References :

Variety of exercise is the key I have found. For example..an exercise called Mountain Climbers.

Mountain climbers are done by starting in a pushup position
and then shuffling your feet in and out so that your knees are
moving in under your chest and then back out to starting
position. It sort of resembles climbing a mountain but flat on
the floor. If you want an advanced version, you can also shuffle your hands 8-10 inches forward and backward in
addition to the leg movements. This really makes it a full
body exercise and MUCH more difficult than standard mountain climbers.

After finishing each exercise, rest about 30 seconds before
starting the next exercise. Rest about 1-2 minutes after
completing each “tri-set” before repeating.

This will give you one of the best ab workouts you’ve ever had
without even doing any direct ab exercises. You’ll see what I
mean after you try it!

Once you get away from the myth that a 1000 crunches and 500 situps a day is all you need to do for great abs, then amazing results can be seen.

The upper part of your "abs" (the upper and lower is only one muscle!) isn't a location where fat is predominantly stored. Fat is normally stored just above the waistline, especially in men. The key is to burn fat – not to focus on your ab workouts.

I think the problem here is that a myth has been going around…

You can not burn fat from your midsection by doing ab workouts. That's the bottom line.

What I'm saying is you can do leg lifts to your heart's desire and never get that "tight" look that you're seeking. The key is cardio and total body strength.

Some things that can help is, well obviously some cardio work, regular strength training and, believe it or not, yoga works wonders…

Yoga is great for balance, posture and core stability which works your abs and legs like crazy.

Anyway, no, you can not burn fat from your midsection by doing ab workouts. This is something that has gone through the media and it just is completely false. The key is workout variety, nutrition, and total body fitness. Have you ever seen an individual with floppy arms and legs (an otherwise untoned body) with a six pack? If the answer is "yes", then that person had surgical implants. on top of their subcutaneous fat.

We all have abs, and they're probably tight looking if you've been working them out. The problem is the layer of fat that's sitting on top of them! Lowering your body fat percentage is the only way to get that "six pack" look.

Even if you can't quite get there, don't get discouraged! Only the minority of individuals have the genetics for creating that "washboard look". But, lowering your body fat percentage will give you some definition nonetheless. My advice would be to skimp a little on your ab exercise and focus a bit more on cardio and regular strength training.

Another thing to remember is that your abs are muscles just like any other muscle in your body. Do you work your chest or legs every day? I hope not, so don't work your abs everyday! They need time to rebuild and regroup just like any other muscle. If you work them every day, or even 5 days a week, you're doing too much. You should only be doing them 3-4 times a week, maximum.