What could prevent a man from being able to sustain an erection?

Often mental state: Vascular disease, diabetes, thyroid, testostrerone defiency etc..Can influence quality and frequency of erections. High anxiety state constricts blood vessels hence reducing blood flow, thus reduces erection quality. Nocturnal erections are often of good quality because your mind is relaxed, enlarging blood vessels and hence flow/erection. Consult doc if you are having trouble. Good luck.

Many things: Including stress and other psychologic issues. But other things including medications, vascular, hormonal and neurologic issues can contribute too. If you find this to be a particularly bothersome issue, consider reviewing this with your doctor to better define what may be the issue(s) at hand for you.