Pearl S. Buck put it really, really well. "Your mind must know it has got to get down to work." Period. End of story. No, actually, beginning of story. Stop woolgathering, stop waiting for the right time (or the right pair of socks, the right cup of tea, the right chair to sit in, the right time of day...) and just write.
That's what I did today. I wrote 1,529 words--most of them in The Color of the Sky--and it was kind of like squeezing blood out of a turnip (although why anyone would want blood from a turnip is beyond me... gross). They didn't flow freely. They didn't come easily. They don't look very good now that I'm rereading them. They didn't even sound very good when I read them out loud to my sister.