You may want to enter a typical day's food into fitday.com and make sure you are getting enough fat. With the low-fat craze fresh in our minds, we Atkins folks tend to try to eat low-fat Atkins, which will leave you constipated even if you're getting enough fiber. I lose best on Atkins when I'm eating 60 - 70% of my calories from fat, 20 - 25% of my calories from protein, and 10% of my calories from carbs.
In addition, make sure you're drinking lots of water.
Best wishes!