What is my problem the urologist say nothing?

i am 28 y old.I have seen a urologist and he has tested me and said that i have no problem. But sometimes (may be 1 to 2 time ) in a week i do not have any morning erection or the erection becomes flaccid very soon . What could be the reason? I am a virgin and if i have a touch with a girl i have erection, but i am tensed with this morning erection plz help...