The middle of a story often drags. This is so common that “problems in the second act” is becoming a cliche movie criticism. You’ve set up your hero’s goal, but if they go and achieve it immediately afterwards, it seems trivial. In order to make the final conflict seem significant, you need the hero to try some things that don’t work, or go on some quest to gather necessary allies or weapons, or have some interminable training montage or something. The problem is, this middle part exists for no other reason except to pad for time in order to build anticipation for the finale. As such, it is often very boring.
If you poke around in script writing advice blogs for even a little bit, you’ll quickly come across the recommendation that you add a sub-plot so that you can resolve it here in order to keep things from seeming too dreadfully dull. Maybe that’s a good idea for screenplays. I don’t know, because I sneer at the constraints of a 90-120 minute movie and instead embrace the unrestrained decadence of novels and television, where a story having two or three times the amount of content as a movie is considered a minimum. My recommendation is that a mere sub-plot isn’t nearly enough. This section of a story is referred to in the Hero’s Journey as the Road of Trials, and if you do it right, being trials plural is usually beneficial.
There is more to this than simply having a hero overcome a bunch of obstacles, though, and some variation on just doing that is often the problem. The hero beats up a bunch of mooks or learns a bunch of secret techniques, and it doesn’t really amount to anything except time-wasting before the final confrontation. What needs to happen here is that a theme needs to be explored from multiple different angles, with each specific perspective being given its own mini-arc (if you really want to squeeze this down to a single event so you can fit it into a screenplay, you can have your beginning ask a question, give a wrong answer in the middle, and then the correct answer at the end).
Here’s an example: the heroes are a plucky band of rebels resisting an authoritarian dystopia. Out on the fringes, a resistance has formed, but it’s scattered and broken into pieces. The good guys go about to different rebel factions to convince them to set aside their differences and/or beat them into submission, and then they’re able to operate more effectively and topple the evil empire. Thematically, your story starts with a premise and a question: Authoritarianism is bad, so what would be better? At the end, you’re going to assert an answer, which is probably some kind of centrist liberal democracy or whatever. In the middle, you explore other answers, including how things might go wrong. You have a rebel faction who are anarchists and prone to constant infighting because no one will ever compromise any of their principles for the sake of getting along and working together with others, which means they constantly splinter into smaller and smaller factions. You have another who believe that absolutism is great except we accidentally gave the big chair to the wrong guy, and wants to put their cult guru on the throne, and despite lots of surface level differences they are basically the same. You can have a third that has too direct popular control over policy, that results in demagogues achieving total power because the system is too easily to completely rewrite overnight so long as you can convince 70% of people to agree to it, and one that’s bogged down in bureaucracy to the point where no one person knows how their leadership even functions or why they’re enforcing the edicts that they are, and so on. Each rebel faction represents a specific answer to the question of “what would be better than this evil dystopia,” and learning from the ways in which they are wrong allows our protagonists to do it right when they finally stab the Glorious Leader in the face.
That specific example might not appeal, but there’s plenty of other questions with multiple possible answers to explore, where it’s not immediately obvious what the drawbacks of one answer or another are. What is the nature of justice? What does it mean to love someone? What kind of bear is best? This middle section is your opportunity to explore multiple possible answers to the question and demonstrate their flaws before presenting the one you believe to be correct at the end (and if you don’t like providing definitive answers to difficult questions, you can always make interactive stuff, where it’s the players’ job to determine which answer is correct, not yours).