I also have this issue with the erection being better if I am laying down. Mine also goes away immediately when I get up.
I notice that any sexual position other than me laying down and girl on the top tends to make my erections weaker.
I’m not sure if this is a blood flow issue or not. However, I did have an ultrasound done of my testicles and prostate and I was told the blood flow to the area was good. So I believe there is something more to this issue than simply blood flow.