Why does my penis not get hard when i watch porn?

So the other day i popped open a cold fresca and sat on my couch channel surfing. I got to channel 493 and suddenly a big busty lady comes on the screen moaning away while some ***** is rug munching her. So i says **** it, pull my junk out and start polishing the old septer of love. But nothings happening, just a flacid little earth worm that looks like its struggling for life. Does this happen to anyone else?