It happens to me sometimes, but mostly it's just a matter of not warming up properly. You use muscles when you draw, so it makes sense to warm them up before trying to punch out a masterpiece! I do it with gesture drawing, the pixelovely gesture drawing site even has a "class mode" to help with this. They start with shorter intervals and then get longer, for as long as you have specified.
And then there are days where no amount of gesture drawing can make me feel good about what I put on paper... That's when I do something else until the mood comes back, since I'm not bound by a comic schedule yet.
Edit: This post must be cursed. I just spent 1h+ trying to do gesture drawings but I ended up hating them so much I had to take a break, haha.