I think I know exactly what you're describing, and I'm still getting the hang of plot development! It's so easy for me to write down an enticing first chapter, and then wonder..."Now what?"

For me, what's been helping is sitting down and plotting the narrative in advance. I used to be a "pantser" - what they call writers who just sit down and see where the story takes them. But that would always leave me in that limbo above, where after a certain point the story just seems to drag and I don't know where to go next.

Because one of my goals is to have my stories bring in an income, I strive to write "to market". This means I look at common tropes and story structures in my chosen genre and follow that formula. So usually the minimum I have to get started is the genre and some semblance of setting: eg "I'll write a science fiction planetary adventure with romantic elements" - and I will have a beat sheet for that genre.

However, that really only provides me with a basic beat-skeleton, the plot details and conflict etc still need to be clarified within that skeleton.

For that, I've been using Plottr and planning out chapter by chapter and scene by scene. You don't need this particular software to do it, but I find it makes it easy for me to create a simple timeline of the book from the perspectives of all main characters. Because I am writing in a series and timelines intertwine between books sometimes, I also use it to keep all of those major intertwined plot points in sync. Plottr also contains several templates for that earlier genre-specific formula I described, so you can flesh out the plot with the major beats in mind.

For me, the hardest part is the climax. Coming up with something that feels major enough to constitute the main conflict in the book gets a bit daunting, but I'm improving with practice.

If I'm not sure where to take the story while plotting, asking the main protagonist(s) questions usually helps. The character's final goal and desire can really clarify in what direction to take the story. Examples of things I've paused and asked while plotting (with example answers):

* What does this character think they want? - To have an exciting adventure.

* Why do they want this? - To prove to their friends and family that they're exciting, shedding their image as a boring, predictable person.

* What happened to make them feel the need to do this? - After years of being known as a dependable, responsible person, they got dumped by their fiance for (allegedly) being boring and unadventurous. They want to prove to everyone, and most importantly _themselves_, wrong by...volunteering for a mission to an alien planet.

* What is the character afraid of? - Getting sucked into a situation out of their control, where they feel unsafe, and being too weak to handle it.

* What is this character's true nature? (This can get revealed to you as the author, but not _necessarily_ to the character): The character thinks they want an exciting adventure, but really they want more of the same safe/controlled scenarios. They only volunteered to go because they were assured it'd be perfectly safe and temporary. They want an adventure with guardrails, and that continues to keep them in their shell: exactly the kind of person everyone thinks them to be.

* How does this character grow through the story? - by being forced out of the scope of what they THINK they want, into a REAL adventure, and having no choice but to face it and get out of their comfort zone to survive.

* What does the character learn? - That they can handle a real adventure, and can overcome greater challenges than they thought they were capable of.

Often as the above answers appear, so do potential plot points to exploit the character's desires, misconceptions, and the direction in which they're meant to be growing. If the character thinks they want a safe adventure, throw something that looks safe and predictable into their path and have them jump at the opportunity. Then have the adventure turn out to be not so safe after all, and now it's too late for them to turn back. How will they react when the first wrench is thrown into their plans? And who throws the wrench? And how do they react to the thrower: do they hate them, or do they grow to admire them and grow from their example? What does the thrower learn from _them_? (This is where you might start asking questions of the other character)

I think you get the idea. Basically, for me it's mostly about asking questions about the characters while staying within a framework of successful story structures in my chosen genre.