Poor absorbtion can be due to SO many things!!! Inflammation, allergies, candida, lack of essential vitamins and minerals, lack of bile flow, digestive enzymes, and poor eating habits overall.
Start by reading the Increasing Digestive Power notes and follow those guidelines best you can! :) Cleaning the intestines is ALways a good idea, as is pinpointing food allergies and avoiding them for 3 months until the body "resets" itself.
It takes time. Sometimes, it's just the way your body is, part of your Dosha. I always have and probably will always have teeth impressions and light nail ridges on a few of my fingers - my index fingers are really nice and smooth, interestingly - it just goes along with being Vata. So I follow my guidelines 80% of the time and things work out fine. :) I must admit to some late-night snacking oh dear! :)