Are dishes, utensils or dishwasher racks blocking the dispenser which kept it from opening fully?
Items blocking the detergent dispenser will keep it from opening. Make sure water action can reach the dispenser.
Is the detergent old (when using powdered detergent)?
Older detergent, exposed to air, will clump and not dissolve well, which causes the dispenser door to stick to the detergent. Replace detergent if necessary.