I cannot stay hard during masturbation

I do not understand why I cannot stay hard, during masturbation. I start off hard, then I lose my boner, and I go almost semi-flaccid. I can obtain a boner when looking at an attractive woman, in real life, but when it comes to porn, it just doesn't work. I have had wet dreams before, so, I do know that I can masturbate, and sperm will come out; the primary issue here, though, is keeping my hardness.