Possible lack of enzymes. Try taking some probiotics and digestive aids. I HAVE to take them EVERY time I eat or my body will not digest anything and I will not have a bowel movement for months. I have no "good stuff" and a bad overgrowth of "bad stuff", including E. Coli in the digestive tract.

I would suggest not wasting your money on cheap digestive aids. Find a high quality one, if you can. I tried about 20 different ones, before I finally found one that worked.