A larger penis, by normal standards, does not need too much more blood than a penis smaller than it. Just for jokes, lets say a 10" x 7" penis used 1 pint of blood and a penis half it size uses 1/2 pint, there really would not be a HUGE blood difference, erection wise. The body is very proficient at sending the proper amount of blood to the area of the body that needs it. My penis gets so hard it is difficult for me to push against my erection level. Even though Shane is much thicker than me, my greater length would compare, volume-wise to his penis (or close). I get stone erections and Shane gets mediocre erections.
A penis would need to be GIGANTIC for the body to not have enough blood to supply an erection but I think it is safe to say that there are not too many guys running around with a penis the size of a roll of paper towels
Shane as well as Raul from BangBros., along with a few other guys have issues getting completely hard. This may be psychological but in some cases it may be physiological. In the cases of Shane and Raul I believe it is
physiological. They both are able to get erections instantly, they obviously have no insecurity issues so it is east to surmise that they have physical issues with getting strong erections.
The physiological reason this happens, mainly, is vascular restriction. This is when a not enough blood makes it to the penis during an erection. The causes can span from smoking and Priapism to actual vein depletion or clogging of the veins that supply the penis with blood.
There is a surgery that can correct this called Penile vascular revascularization. It is more or less an exploratory procedure in hopes to find the blocking issue and repair of the problem found. As with any surgery it is dangerous and it may not help at all.
In many cases, through my extensive experience, many of these "clogging" issues can be corrected through Penis Enlargement, mainly jelqing. The forceful nature of blood flow in Penis Enlargement can help destroy the blockage and return the penis to normal function.
I hope this helps