Probably just out of practice. Even during the years when I wasn't writing SF, I was still writing many pages per day, averaging perhaps 10-20 pages a week.
From the time I started writing in 1979 until graduating from high school, I spent a lot of time writing--first longhand, then on computer once I got my C-64 in 1983. I spent more time writing than doing anything else, and after graduating from high school I spent a lot of time in the wee hours of the morning writing, writing, writing.
Going to college--in 1996 I wrote 250,000 words in a summer, after I graduated, but I hadn't slacked off while in school, either. And even while working full-time as a tech writer I still didn't stop.
Only in the last few years has it become difficult for me to write, and I don't know why. But "difficult" is relative, because the hardest part is still getting the text out of my brain, not thinking of what is to be gotten out.
Today I added a mere page and a half to what I wrote yesterday, about the guy on the agricultural planet, and I feel as if I'd been pounding the keyboard for more than eight hours.
Well, no matter, I guess. Just have to keep at it.