Loss of Erection When In Certain Positions

[Anon3718]

I'm a 21 year old college student, and I've noticed that I tend to lose my erection when I'm in the missionary position.  I am fairly new to sex, and I generally masturbate while lying on my back.  I have no problems getting hard - I get that from simply laying next to a person, and I stay that way as long as I'm laying down (on my back, side, etc).  I have been mounted while I was on my back, and had no trouble staying hard.  However, the moment I move to the missionary position I lose my erection.  I haven't tried any other positions yet, so I'm not sure if it's this specific position or what.  I figured by pointing my penis down I'd actually increase blood flow to it, so I'm confused as to why I can stay hard on my back, with my penis pointing up, but then lose my erection when I have it pointed downward...

I am taking adderall for ADD, but like I said, I don't have a problem staying hard, so long as I don't move from the lying down position.  Does anyone have any ideas as to why this might be happening, and have suggestions on how to correct it?

[Anon3718]

I'd like to add that I have no trouble "lasting" when it comes to fooling around.  I have no trouble lasting (while hard & on my back) for 30 minutes or more when being stimulated by a hand or mouth, and last even longer during penetration.

[Abhinash]

Dude same here, did you find out why? Do you have low back pain? Or leg pain?