You lower abdominals always seem to be the hard to reach place.
To have a full six pack you need to work them.
Doing hundreds of crunches a day will not cut it.
They are ineffective anyway.
Here are some of the most powerful lower abdominal exercises.
Reverse Crunches - These are very effective.
they don't for equipment either so you can do them at home if you like.

• Start by laying on the floor.
  
• Lift your legs up and bend your knees so your calves (lower part of your legs) are parallel to the floor.
  
• Rest your arms on the side and pull your legs back until your knees just about touch your chest.
  
• Bring them back to the starting position and you have done one rep.
  

Hanging Knee Raises - These are great for working your arms as well since you are hanging.

• Grab on to a pull up bar with your palms facing away from you.
  
• Keep your body straight.
  
• Then without swinging lift your knees up to touch your chest.
  
• Then gently lower them.
  

Tip: It takes practice but once you have mastered them you can make them more challenging by lifting your whole legs so they are about perpendicular to the rest of your body.
Swiss Ball Pull ins - These are a little challenging at first like hanging knee raises but very effective.
You will need an exercise ball.

• Get into a push up position only this time rest your shins on top the exercise ball.
  
• Pull in the ball so that your knees just about touch your chest.
  
• Then push them back out to complete the rep.
  

Do not underestimate how effective these lower abdominal exercises are.
You will start getting results within a couple of weeks.