Agatha Christie stated in her autobiography that she'd get a "broody" feeling when she was between books, or when her writing wasn't going well.
The last month, I've had a serious case of the broodies. I was working on the last 3 chapters of DOUBLE CROSS, the second novel in my Twins mystery series. As I was tying up loose ends, I discovered a few loopholes.
Writing a book is like building a house. You need a good structure and you need a floor plan that flows. The rest is cosmetics and can be changed once the first draft is complete. But if you're putting on the roof and realize walls need to be moved around or eliminated, it's easier (for me, anyway) to go back, make decisions on plot changes and implement them right away.
What brings on the broodies is losing momentum. Momentum is what keeps a story's energy level high. Most writers handing out advice will tell you to keep writing no matter what. Save rewrites until your first draft is complete. Absolutely sound advice, yet for me, it only works for cosmetic changes. Easy enough, once the house is built, to adjust the paint color, or even move non-load-bearing walls. Structural repairs are different. I can't keep working on a building in danger of collapse.
Going back to fix problems is part of the craft. Sometimes it's a pain, but more often it's a good challenge, even fun. You see a better way to tell the story, and that's always satisfying.
The good news is, the loopholes are gone and yesterday I finished the first draft. I celebrated by turning off the PC, making a big pot of soup and watching a Ghost Whisperer marathon on TV. Now I'm ready for rewrites.