you could have any number of things going wrong in there.
you could have spinal misalignments, if a certain nerve is being squished by a spinal subluxation that nerve can't send the impulses to your colon, so it won't be working properly. see a chiropractor to fix that
you could have a yeast imbalance in your DT (digestive tract) go to a health food store and you can buy a candida cleanse or something similar, it will detox your body of that stuff
you could be lacking probiotics (good bacteria) in your DT, that sometimes is related to a yeast imbalance, health food store has probiotics in capsule form in refrigerator section (keep cold always)
your lacking fibers, if you don't have any fiberous material in your body there wont be much feces to pass through. fiber comes in two forms, soluble and insoluble, examples or soluble fiber is oatmeal, wheat germ, psyllium and insoluble is from plant matter like broccolli stems, etc. when you lots of fiber through you gotta drink lots of water or its not as effective.
you could have a blockage in your colon , that to is related to probiotics and sometimes yeast imbalance and sometimes fiber and water intake. if you don't have the proper organisms in your DT to break down all the food you eat plus if your not getting any fiber then it will just sit there rotting and compacting itself coupled with a misaligned spine there is no perstalsis (movement of colon walls) forcing that food along
if you want to check for blockage just gently press along your DT feeling for any hard or painful spots. the large intestine starts roughly at your right side pelvic bone then comes up to the bottom rib area and then straight along to the left side, then down to the left pelvic bone and little crook into the rectum an then bammo yer cornhole. if you find one, gently rub it in the direction it supposed to be going, you may feel nauseas but believe me its worth it to get things going again
or you could have a food allergy. do you eat lots of wheat? or dairy? maybe you are the gluten or protein in wheat, its fairly common (celica disease). i myself have a really bad allergy to wheat gluten so i have to avoid EVERYTHING with wheat in it, believe me that is frikkin hard. although it has helped me to learn about healthy food and lifestyle choices.
you can get a test from doctors for celiac and wheat allergy.