The inability to write what you’ve outlined plagues many of us, and it has many potential causes, some of which have workarounds, while others don’t. Here’s just a couple:
1. You don’t know your characters well enough when you outline, so when you write them, they make unexpected choices.
Solution: Develop your characters through writing before you outline. You can do this by role playing online, or adding them to short stories or by writing random scenes from your outline. (And keep in mind here, that sometimes thinking about your characters and filling out forms or taking notes will help, and sometimes it just creates a misguided illusion which will change the moment you start writing. Characters can be funky like that.)
2. You’re focusing your outline too much on where you’re going and not why you’re going there, so when you write, your story goes in the wrong direction.
Solution: Put more time into developing the motivations and goals of the characters and the initial conflict, making those tight enough that they can only lead to a certain outcome.
Instead of viewing your outline as a series of events (“I want the three main characters to have an argument and then they all go to the beach”), look at what’s forcing these characters to do the thing you want from them (“I know the three main characters all have vastly different opinions on what to do with the stolen money which are deeply rooted in these specific parts of their past and this is driving a wedge between them, but if they don’t go to the beach together they will lose out on their old chance to question the mysterious person who disrupted their lives and forced them together.”)
3. Your story is trying to tell you what it wants to be, but you’re clinging to your old ideas of it and so can’t enjoy the new version.
Solution: Just don’t outline. This isn’t something I recommend most people do, but occasionally there are writers who create better stories when they don’t try to preconceive what they want out of the story ahead of time, and just let it take it’s natural course. You can always edit things around later!
You can also mix this with solution two, and outline the inciting conflicts, goals, and internal struggles ahead of time. By carefully devising characters who clash and placing them in situations with high stakes vs rewards which drive them to work together toward a common goal against an opposition they are thematically and externally bound to, you’ll have a solid foundation upon which to build your story.
4. You’ve done everything imaginable and still can’t get your rough draft to match your outline.
Solution: Edit or rewrite until you get where you want to be! Books aren’t finished in one, or even two or three drafts. They need a lot of revisions, and some of those can involve correcting plot dentures and character arcs and so-forth.