Keeping an erection while laying down.

And the title would be my problem, I can keep an erection fine any other way, but when actually laying down(on my back), I just can't keep one, whether it be masturbating, or getting a blow job or whatever. I can get a bit semi-erect, but that's it. Anyone know what would cause this?

 

An extremely low blood pressure.

 

Madcap - it could be low blood pressure (have this checked by a doctor), or it could just be something in your head which is preventing the nitric oxide from flowing regularly from your brain to your penis, which opens the capillaries in the corpus cavernosa sacks in the shaft and allows blood to flow in and stay there. Try to think about something else when you are masturbating or getting a blowjob on your back, other than "I hope i don't go soft". Try counting the strokes.