If 2 table spoons of apo lactulose doesn't work for constipation, what could be the reason ? and if there is no over use.

Hard to know: Lactulose is a synthetic hydrocarbon hydrolyzed in colon by bacteria to Lactic Acid and amounts of formic and acetic acids, resulting in increased osmotic pressure and acidification of the colonic contents. It is used in chronic constipation. If simply increasing the amount of liquid in the stool does not relieve constipation the stool may be too compact or mechanical obstruction may be present.