Two from my own experience: 1) Finished story, off to beta readers. They all want me to write one particular scene - which I am finding very difficult to write. Could be worse, if I had no interest in writing said scene - meeting of two main characters, which at the moment happens off-page...). 2) when you're writing a scene, or worse yet, whole story, and realizing YOU are getting bored...time for a major re-write!
As Kathrine Rusch writes in one of her recent blogs, writers need to be storytellers first. If your story is boring, no one will read it beyond a few pages, no matter how finely crafted the words on the paper are... (admittedly, I stop reading even interesting stories if the spelling, grammar, etc. Is really very bad, but if the technical aspect of the writing is average, I'll put up with a certain amount of errors if the story keeps me interested.)