I was wondering if you could go into those scenes that aren't so definitive. For example, sometimes I have scenes I can really visualize because something physical and tangible is happening, but then there's always that time of reflection afterward I have a harder time "seeing" these in my mind's eye which can sometimes make them harder to plan out in scene form, meaning, figuring out the beginning, middle and end, etc. Because, in reality, they're discussing or contemplating whatever disaster has just befallen them. So, I have trouble constructing this into a viable "scene".
That time and reflection afterward is traditionally called a sequel. It's not an actual scene, which is why treating it as such doesn't work. Sequels have no goals to move the story forward. This is the time when your character reacts to what's just happened, and makes a decision that moves the story to the next scene. But they also don't need to be very long to work.
I'm going to take a step back and look at the big structure picture here, since I think it's easier to understand sequels if you see how they work with scenes and story arcs.
Basic scene structure goes something like this:
Protagonist has a goal. They'll act in way to achieve that goal throughout the scene. They'll either get the goal, don't get the goal, get the goal but there's a catch, not get the goal and make things worse. Scene ends, because the goal has been resolved in some way.
Then the protagonist reacts. They'll have an emotional reaction, think about what they just went through, and then try to figure out what to do next. This is the sequel.
What they decide to do next is the goal for the next scene, and it starts all over again. Sequels are important in directing your story because this is where the protagonist does a lot of decision making. The emotional state they're in when they make it determines how readers will feel moving forward. If the protagonist is scared, readers will be too. If the protagonist doesn't much care, readers won't either.
There are smaller scene goals that drive a particular scene, and there are also larger goals that drive an entire story. (More on that here) Larger story arc goals are usually the big things your protagonist needs to achieve a win from a story standpoint (defeat the big bad guy). Smaller scene goals are the steps that lead up to the big goal victories. Sequels are the glue that holds it all together and remind you why this is all important in the first place.
For funsies, let's see how Bob is doing with those zombies and put this into context.
Bob is being chased by a few zombies. The scene starts with him running, and you could say the scene goal is "Bob wants to escape the zombies." But just escaping is more of an overall story goal for Bob, as he's been doing this all along. To resolve the story and win, he needs to escape for good. Right now, he's has something specific in mind to defeat these particular zombies, and that will drive this particular scene. "Bob is trying to get to the motel where his extra shotgun is, so he can escape the zombies by killing them." Bob can't achieve the bigger goal resolution (escape for good) until he achieves this smaller goal (kill these zombies).
This scene will end when Bob gets to his shotgun and one of the above four things will happen.
1. Bob gets the shotgun and kills the zombies. (he gets his goal)
2. Bob doesn't get the shotgun and the zombies eat him. (he doesn't get his goal)
3. Bob gets the shotgun, but he has no shells and has to find some. (he gets his goal, but there's a catch)
4. Bob gets the shotgun, but shoots himself in the foot as he aims for the zombies. (he gets his goal, but makes things worse)
Options one and two stop the story cold. There's nothing driving it forward anymore, because no new goal is set. Option three gives Bob something solid to do that puts him in more danger and moves the story forward with more risk. Option four makes things worse, raises the stakes, and gives Bob something to do to move the story forward. Bob's had it too easy lately, so let's give him option four. He shoots himself in the foot, but he does take out one zombie in the process.
So what's Bob going to do next? Have a sequel.
Sequels don't have to be long. In fact, they're better when they're shorter since there's nothing driving a sequel. They will stop the story cold if left alone too long.
Bob's sequel could be as short as: "Ow! I'm such a moron. Where's the dang first aid kit?"
He has a reaction (ow). He reflects and thinks about his situation (I'm such a moron) and then he makes a decision. (where's the dang first aid kit?) New scene starts with Bob trying to make it to the first aid kit before he gets eaten by more zombies.
Now, if Bob had shot himself at the same time he shot the last zombie, he might have a longer sequel. He might take a few paragraphs or pages and reflect on his situation. How he got into this mess, what he needs to do to get out of it, what is still in his way. I've found that with longer sequels, it helps to add a sense of tension or impending doom to keep the story moving while your protagonist has a reflective moment.
actively trying to eat him, keeps the story tense and moving while he thinks about his situation. These are also good moments to remind the reader what's at stake or any overall story goals. If Bob fails here, he fails the whole book.
It's also good to remember that scenes can span chapters. Just because a chapter ends, doesn't mean a scene has to end. You can have multiple scenes in one chapter, one scene per chapter, or mix it up. Revelations are good for scenes that span chapters. You can end a chapter with a bit of shocking news to keep the reader turning the pages, and continue with the scene the next chapter.
Mix and match is a great way to keep readers guessing, because there's no set formula. If terrible things can happen anywhere in the chapter, they won't think "gee, this chapter is almost over, I bet something shocking is about to happen." They'll just keep reading to find out what comes next.
And the sequels are often where they realize those shocking truths or discover how bad something truly is.