It's likely a combination of all those things. Maybe this isn't the case for you, but a part of it I deal with regularly is that I don't know everything about the story before I start writing it. As the draft develops my mind gets filled with the history of my characters and world and it becomes almost effortless to drop in the right details at the right places, to make references to past events that don't require any explanation ("it was like that time in the lumber mill").

It's perfectly fine for the earlier parts of the book to be a little awkward because they lack this established background, and anyways as anyone who's ever gotten into a romantic relationship later in life, it's always awkward to bring someone into your story and catch them up to current events so that you can start writing the future together. Here's how I become a lawyer, how I wound up in Kentucky, how I came to find myself in the middle of a catastrophic conspiracy involving the President. Now we can marry.

It's very unlikely to go smoothly yet most readers don't even care that much as long as the premise is interesting and John Smith seems like someone they can root for. Almost every book I read, I can pick out the point where the author changes gears from "setting up all this bullshit" to "here's the fun part I wanted to show you all along." I can see the transition even if it's in the middle of a chapter or between two paragraphs. Some authors hide it better than others and only masters can disguise it almost entirely, but it's always there whether you perceive it or not.

And just like any other writing obstacle there's a tool you can use that, while it won't cure the problem, can at least remedy it somewhat. It won't make you like a master but it'll get you to the point where you can be satisfied enough to finish the damn book.

I started this technique after my first couple manuscripts, and it was this: I'd write everything out like normal, making the first chapter or two the boring "here's my life before the candy store blew up" nonsense (For whatever reason we are tempted to lead with the grey and not the red, it's just common sense and it's bad even if it's forgivable. Don't fight it just deal with it.)

Then when I finish the first draft I go back, select those two chapters like cutting out a tumor, and drop them into a different document entirely.

Now the start of my story is the former chapter 3 and suddenly things are a bit more interesting right off the bat. Of course I can't just throw away all those important details from the first two chapters and I definitely still need to write a paragraph or two so the reader isn't wondering how the fuck we wound up in the middle of the desert fighting mutant gorillas, but now all that boring shit is safely confined to a space where it can't harm threaten to bore my reader to death any longer. It simply serves as a bank of knowledge I'm obligated to withdraw and place in the 2nd draft, somewhere, ideally wherever it feels natural. This isn't exactly in medias res and it doesn't mean you have to resort to flashbacks. Sprinkle all that boring shit in like vinegar in a dish that's too sweet.