No matter how much we think we know about our stories before we start writing or outlining, many of us inevitably run into a wall or two when it comes to our plots. Maybe it's writer's block. Maybe our characters won't do what we want them to. Maybe we've had to incorporate something into the plot once we started writing that has derailed all of our carefully constructed outlines and plans. It's easy to get discouraged when you hit that wall, and you may be tempted to throw your hands in the air and give up. Don't do that. I want to offer some strategies you can use to break down that wall and get your plot back on track.
Strategy #1: Eliminating Resources to Create Conflict
When you stop and think about it, plot is really just a series of small conflicts tied together to create a larger, overarching conflict. A lack of compelling conflict might be one of the reasons your plot might have hit a wall. In that case, you either need to add some conflict or just spruce it up a little. Sounds simple enough. The problem with this is that, if you're anything like me, it can be hard to generate ideas for conflict out of thin air. And when you do, sometimes the ideas you come up with feel contrived or forced (probably because the very nature of forcing yourself to churn out ideas leads to ideas that feel forced).
Instead of just jumping straight for the conflict, take a step back. Conflict arises from the problems, obstacles, and challenges that your characters encounter, so what you really need to do is introduce some of those things. The rest should follow naturally.
First, make a list of all the resources or assets your characters have to help them achieve their goals. These might be physical items such as weapons, magic, a spaceship, their dashing good looks, and so on. They might be more abstract things like the close friendship between your two protagonists, the secret support they've been receiving from the director of their agency, or their unfailing motivation and determination to succeed. Think outside the box. The longer your list, the better; it will give you more options later on.
Now, start thinking about how those things might be taken away from your characters, and how losing those things might lead to conflict. Maybe your protagonist has a group of loyal supporters dedicated to helping him destroy an evil magical object. But wait! How much more interesting might things get if the members of that group were killed or captured or just decided to pursue other goals, leaving the protagonist to struggle on alone? (The Fellowship of the Ring) Or, let's say your protagonist is a special agent on an unsanctioned mission to kill the leader of an alien race on another planet. It's a dangerous assignment, but they've managed well so far due to the secret support of their agency director. Unfortunately, the director's bosses find out what she's been up to, so she's forced to withdraw her support. This leaves your protagonist trapped on a hostile alien planet alone and without any kind of outside help or information. Can you imagine all the possible conflicts that could arise in that situation?
You don't just have to limit yourself to removing your protagonists' resources, either. You can create interesting conflicts by using this method with your antagonists, too. Maybe your driven, tenacious antagonist finds something that causes her to question everything she's been doing up to this point. She starts doubting herself and her motivation falters. As a result, her underlings begin to distrust or question her. How might that cause conflict for her? The effects might even reach further than her inner circle to affect the protagonists as well, whether for good or bad. There's nothing I love more than a well-developed, three-dimensional antagonist who is more than just an "evil" archetype. By giving your antagonists setbacks and allowing them to deal with their challenges in an authentic way, you not only add more conflict to your plot, but you create more interesting antagonists.
You don't have to take away everything from your characters. I mean, if you want to, go for it. Just try not to put your characters in such an impossibly dire situation that you have to resort to a deus ex machina to give them the ending you intended. That being said, I do encourage you to brainstorm from your list until you've exhausted all possibilities that make sense for the story. You don't have to use all of them; some will probably be utter crap. But at least going through every option will give you a good idea of what will work best. From there, it's a fairly simple matter of deciding consequences, making connections, and stringing things together into a coherent conflict (or series of conflicts).
The idea here is pretty straightforward: You want remove your characters assets and strengths, which will back them into a corner. Everything they do to get themselves out of that corner becomes part of your plot.