What causes the feeling of incomplete defecation?

It depends: It may not be anything for certain but a check up by your doctor is necessary in order to be sure that there is no growth or tumor inside your rectum which can give you a feeling of incomplete evacuation. So get a rectal examination and seek advice from your doctor.He may advise you to have a endoscopic examination of the rectum.

Different things: Different things like- if you have constipation, or if there is anything obstructing-like a growth/mass, if the rectal muscles/anal spincther tone is decreased/weakened- you can have the feeling of incomplete defecation. Go to see your doctor and get yourself checked.