Subtlety can sometimes mean the difference between a passage that works and one that falls flat. One word change, one shift in perspective, and everything's different. These can be hard to spot sometimes, which only adds to the frustration. If you're struggling with a story right now, and getting good feedback, but just not quite landing the book anywhere, (or having folks tell you it's good, but not great, or lacking something they can't put their finger on) perhaps take a closer look and see if there's a subtle reason that's holding the work back.
Are you writing a romance with a mystery subplot, or a mystery with a romance subplot? Both books can look quite similar, but what one genre expects is different from the other. There might be little things you're doing that nudge it toward the wrong side and makes it feel off, even though no one can tell you why. Check your plots, structure, tone, look at the tropes of your genre and make sure you're in line with the norm for that genre. Make sure that the story you want to tell is the one actually being told on the page.
Sometimes there's a fine line between narrative that's in your protagonist's POV and the author describing what's happening to a character. Look at how you offer information to the reader. Is it technically accurate and well written, but feels as if anyone could have said it, or does it sound like your POV character? Do they sound as if they're experiencing these events as they happen, or simply relaying them as if they were describing a movie or sporting event?
Since you know how events will play out, it's easy to write a scene that shows your protagonist doing all the things they need to do to succeed in that scene. This can sometimes give the scene a sense that there's no actual opposition to the protagonist's goal. All the pieces are there, but the feeling that the antagonist is really trying just isn't there. You're not showing how the protagonist struggles to win, you're showing how they overcome obstacles to win. One leaves you with a sense of uncertainty (they struggle, and they might lose), the other leaves you with the sense that they're just going through the motions (here's how they overcome this problem).
Telegraphing the Text
Check your descriptive action scenes. Are you having your protagonist state their motivations before they act? This can give the text a detached feel, as if the protagonist is explaining things after the fact and not actually participating in them. Look for sentences like...
But when she tried to run for the door, Bob stopped her.
Make the subtle change to...
She ran for the door. Bob stopped her.Tiny change, but see how the second example feels more active and immediate?
Stating, Not Showing
The tiny word "to" can do a lot to steal the sense of immediacy from a line. In most cases, "to do" something is telling the reader what a character intends to do, but doesn't actually show that character doing it.
Bob stepped out on the balcony to check for zombies.In the first example, Bob doesn't act. The author is telling you Bob plans to act. Second example shows him acting.
Bob stepped out on the balcony and checked for zombies.
There are, of course, exceptions to this and places where this is perfectly acceptable, but "to" is a good word to check on to make sure you're not inadvertently telling when you want to show.
Other words to check on...
Bob could see the zombies in the distance.Try... Bob saw the zombies in the distance.
Bob was expecting zombies to come out from every corner.Try... Bob expected zombies to come out of every corner.
Bob noticed the bushes behind the car were trampled.Try... Bob spotted trampled bushes behind the car.
Bob watched the zombie rip George's face off.Try... The zombie ripped George's face off.
There are more words like this, but hopefully this is enough to give you the idea of what to look for. Search for things that describe action and turn them into phrases that show action.
Change of Subject
Check your sentences for places where the subject of the sentence isn't the person you're talking about. This can make a sentence feel flat.
The footprint gave Bob the feeling that a zombie was close by.The footprint is the subject here, but the footprint isn't doing anything, so the sentence just lies there. A subtle shift to Bob, and suddenly the sentence has new life.
Bob saw the footprint and shivered. A zombie was nearby.Little things can make a big difference, and sometimes they're so small we don't even know they're there. But we often instinctively sense them and know something's wrong, even if we can't say exactly what. Look for the subtle things and train your eyes--and ears--to pick up them and make them do what you want them to do.