Writing a book requires application - time and effort. It's not easy to get 80,000 odd words on a paper (And yes, some of them are odd. Stop sniggering at the back). But writing is actually a lot more than this. It's editing, proof checking, fact checking - of course - but there is also the need for 'headspace' . This is the nebulous bit, and often the one that causes the most problems. Scribbling in a notebook, sitting at a keyboard, revising a manuscript - all these are visible. You are doing something, and seen to be doing something. But none of this will happen unless you know what you are doing before you start doing it. Even those who only do minimal plotting have to have an idea of where they are going, of the nature of their characters, of the setting, of timescale and relationships, even if these do tend to work themselves out on the page.
All this needs thinking time - and often this is the most difficult thing to manage. If you are diverted by the day job, stressed by health concerns, distracted by family issues - even if you just have workpersons in the house - your head does not have room for the world building that needs to go on, consciously or unconsciously. I can't count the times fellow authors have told me that they have not been able to move forward with the current manuscript not because they don't want to, but simply because they don't have the space to think.
There are ways around it. Some people use music. A well known channel is the 'plot walk' - a walk during which you think about plot. Easy. Jobs and chores that don't require a lot of thought can free your mind to wander. I did a shedload of ironing over the weekend and spent the time while flattening the pyjamas and tea towels trying to work out a complex network of family connections in my head. If she was his cousin, instead of his sister ... All in relation to a will and an inheritance, of course.
I'm definitely being pulled along by those cosy crime stories I have said keep bubbling up.
The bigger the bubbles, the greater the attraction ...