Why Does My Penis Feel Not Strong Enough?

hi, Dr. i have problem my penis not stronge and it is smooth. before 3 months ago, happened to me problem my penis still upstanding mybe an hour and a half. Then, i went to Dr. and say to my there is no problem in your penis. And now i dont know why my penis is not strong. Please Dr. i want to know my penis is fine or happened something wrong.