When I first started planning my novel series, I had a solid plan. I had the graphs, the outlines, you name it. I had a very clear idea of what this story was going to be from beginning to end. I knew what the characters were going to do and when, the formula I'd written, and re-written, had everything mapped. I had it all figured out. Then I started writing. One of the things I love about writing, in general, is that no matter how much time you spend preparing and researching before the actual writing begins, the story evolves with the more understanding you gain about your characters, the world and the plot itself. The point is, your plan is never going to work out for all the right reasons--especially since all those right reasons will help you pinpoint the flaws of your story that are holding you back. How though, will you be able to tell?
1) Pinpoint a real problem that you can solve
Here's an interesting problem I ran into when working on my first draft (which by the way I am working on currently). That plan I mentioned early came with an implied connection between one of my three societies and the monsters that exist in this world to purge all humans left alive. Everything was going swimmingly until I realized that I wasn't doing enough to separate one society from the other two. Why was this society of people more special than the others? What sets them apart and connects them to the main problem everyone's facing in this world? What I had initially definitely didn't help how obvious the connection was and undermined how mysterious I wanted to make this connection. Thus, as a result of this, not only did I have to fix the problem, but to fix it, I had to let this aspect in the story change, evolve into something I'm much happier with now.
2) Channel your inner Eeyore, but not too much
Here's the mindset to get into once you encounter that first time your story's diverted from the plan. Be prepared for things not to go your way but don't depend upon planning to write your story. Realistically, planning only serves the purpose of getting you past the starting line and most of the race. Change and evolution of ideas are what is ultimately going to get you across that finish line. Don't panic if your story completely changes from when you began, either. The most significant piece of advice I ever received was, "as the writer, it's your job to write this story and be prepared to facilitate what this story is trying to be." Your story will have a mind of its own, you're the conduit who's going to get it on paper. Be prepared for your story and your characters to occasionally change their mind as you work through the process.
3) Be the change
Or maybe the opposite will happen. You've stuck to the plan, and everything's gone swimmingly, but now your clear-cut plan's landed you in a bind where you've written your story as methodically as you can, but you're no longer in love with the idea. Everything's set up exactly as you wanted it and there should be no problem. Well, there is one and go ahead and face it now because at this moment you've decided you are disagreeing with your story on a point. That's the beauty of writing, too, sometimes you and your story are going to disagree. You're the author though, and sometimes you need to be the one making the changes instead of your story. Don't worry, your story's feelings will recover quickly.
4) You and your story are a married couple of a kind
Yes, you and your story will have your own separate minds. There's going to be give-and-take. Sometimes we'll have to give into the evolving ideas, other times those evolving ideas will come with the inevitable backtrack. Part of the experience of writing will be finding what ideas you change and what ideas you keep. This, however, is a natural part of the process that will be worth becoming more familiar with when working on your writing.