A lot of authors can power through it. FOr me, I've found if I'm really blocked and I push myself to write on the current project, what I write? Is utter drivel. I wouldn't show it to my betas or editors, because it's just *that* poorly executed. It becomes an exercise in frustration and wasted time. Or I end up staring at a blinking cursor for hours.
I move to a different project. I tend to have 3-4 going on at once anyway, so clearly if one project isn't speaking to me at the moment, it's likely another will. Of course, when you're staring down the barrel of a deadline, this isn't going to work but I tend to dislike writing down to the wire anyway.