The Surgeon General has identified two possible causes for such a symptom.
They are firstly, a general weakening of your prostrate and secondly, an absence of penetrative sex for a period of time.
The solutions as recommended and subsequently documented by him are as follow:-
a. Do pelvic muscular exercise to strengthen your pelvic/prostrate muscle.
b. Have regular sex of 10 times a week for the rest of your life.