Jane Yolen once famously talked about the key to writing being having your Butt in the Chair (or BIC). If your butt isn't in the chair, you're not typing at the keyboard. And you're not writing.
Sometimes just staring at the screen is half the battle. But what about distractions? Kids? Furry kids? Social media? Real life?
My butt is in the chair sometimes, but my mind isn't on the manuscript. And sometimes when my mind is most productive, my butt isn't in the chair.
I get my best ideas, the answers to my manuscripts most difficult questions, when I'm driving or when I'm showering. Not when my butt is in the chair.
The key is getting back to the chair to write it down. Before you forget.