I suffer this too. I usually have a cold tip which doesn’t seem to fully fill with blood when I have an erection. I have periods of a week or so every few months where my penis recovers and it fills up properly with blood, and the tip is hard and sensitive.
It’s quite interesting - when I have these mini recoveries my penis curves differently too. It appears ‘fatter’ and it doesn’t curve quite so much upwards. It stays straighter and is quite rigid. In my most common state, it is thinner and curves upwards quite dramatically, almost like a sufferer of Peyronie’s encounters, except I know I don’t have that as it returns to normal through in a recovery period.
This change in erectile function is interesting. I also notice that the region of my prostate feels fuller when I have these temporary recoveries. Upon returning to my usual state, I feel this ‘hollow’ sensation between my anus and my penis, as though something is missing.
Part of me wonders whether the prostate is actually changing in size and/or shape. Perhaps the reduction in size of the prostate means that the blood flow into the penis or even the accommodation of the base of the penis (the part inside the body near the prostate) allows it to lower and thus curve more, whilst when the prostate is in a more normal state, it alters the degree of blood flow to the penis and helps it maintain it’s more rigid and straight erect shape.