the causes are not much known till now for incomplete bowel movement, IBS(irritable bowel syndrome) may be reason but not always. here I can say probable causes as fallow Two physiological processes that have been identified as major underpinnings of IBS symptoms may be playing a role here. The first, visceral hypersensitivity, refers to a heightened pain sensitivity within internal organs—in this case, the large intestine, the rectum, and the anus. The second is motility dysfunction, the fact that the muscles of the digestive tract do not appear to be operating in a smooth manner, thus interfering with the ability to pass a comfortable, well-formed stool and feel as if you have had a satisfying complete bowel movement. usefull measures : Due to the difficulty in passing stool, it is likely that you have not fully emptied the rectum of stool, thus leaving you with feelings of dissatisfaction and discomfort. Therefore, the primary way to address the problem is to try treatment options for relieving the underlying constipation. Increasing fiber, whether through diet, or with fiber supplements, is a good way to start. Another good self-care strategy is to ensure that you have good bowel habits. Try to schedule a trip to the toilet the same time each day. For many people, biorhythms are such that urges for bowel emptying are strongest in the morning. Allow your body the time to empty as much stool as is possible, envisioning that rectum shape as you evaluate the completeness of the movement. If you believe that dyssynergic defecation contributes to the difficulty in passing a complete stool, you may want to look into biofeedback or physical therapy as a way to ease your symptoms I hope you happy with information kindly rate it. thank you