Likely due to arterial insufficiency. This can be diagnosed with a penile doppler ultrasound. Many have already been diagnosed with it, including myself. I do still have good glans flow, but my issues was more venous leakage (blood staying in) rather than arterial ins. (blood flowing in).
Below are the values you use to diagnose.
Peak systolic velocity of the CA Indicator of arterial influx
Normal: >35 cm/sec
Gray zone: 25-35 cm/sec
Abnormal: <25 cm/sec
End diastolic velocity of the CA Normal: <3-5 cm/sec
Venous leak: >5 cm/sec
Diastolic flow reversal: reliable
indicator of intact veno-occlusive
Deep dorsal vein Normal: <3 cm/sec
Moderate increase: 10-20 cm/sec
Marked increase: >20 cm/sec
Arterial compliance of the CA 60%-75% increase in diameter Ø
Resistive index of the CA Normal: >0.9
Venous leak: <0.75