Have you looked into the possibility that you are sensitive to gluten? Celiac.com has both information pages and Forums. The Gluten Syndrome has info on problems caused by gluten. I lost weight by stages due to maladsorption and a clogged bile system before I was diagnosed with Celiac Disease.
A normal/healthy person will have a bowel movement within an hour after each meal. This reflex is triggered by high acidity in the stomach and if you take Betaine HCl or Hydrochloric drops with meals it usually triggers a bm. I also use prunes, herbal laxatives, a capsule of magnesium daily. The longer feces stays in the intestine the more toxins are adsorbed into the body.