Flat lower stomach!
I have the same problem...unfortunately its not just workouts its diet also. Make sure you eat as clean as u can, aoid sweets/carbs and drink only water
Choose one day a week that you eat/drink strictly fruits & veggies and of course, water. It will give your digestive tract a break from having to break down all the junk we tend to ingest. If you are tracking calories, carbs etc., you can break from it that day also.
Lower abs are tricky. The key is to focus on working out all of the areas around your core (abs, hips, lower back, and butt). You entire core is interconnected.
Keep this in mind: if your lower back or hips are weak then your tummy is going to look bigger than it actually is.
I built my exercise (Core Blast) from workouts I found here and at ab-workouts-guide.com. This exercise is made to work on those specific areas so that not one of them is ignored.
Also, keep eating healthy. Stay focused and above all don't give up. Remember everyday you skip a workout or healthy meal is another day lost to get to your goal. If you need anything, I'm here for you.
Good luck and God Bless!