It’s not always the easiest question to answer. A lot of things go into making the jump from the written word to the big screen. Sometimes it works well, sometimes it doesn’t. Some stories weren’t ever meant to be turned into movies while others seem like they were made for it. Let’s explore some of the reason why a book may not translate well:

1. Scripting issues.
Contrary to some ill-informed beliefs, you can’t just have someone read from the book. You can’t. As a result, a writer has to sit down and turn the book into a playwright that can be acted out. Most often, it’s not the author of the original book that does this and it’s usually not just one writer. As with all things, one writer’s creative interpretations are usually quite different from another’s. This will result in the writer’s ‘interpretation’ through dramatic license of how they think the author’s story is supposed to go.

-A second problem is that while a script may have been sold, it’s not necessarily finished and more writers need to be called in finish the job, more writers mean more interpretations. Sometimes for the better… sometimes not…

-Another problem is a director, whose vision differs from that of the writer. In the end he has the final say and script rewrites can happen here at the director’s behest.

2. Studio Interference.
So you have your story, it’s great, people love it, and it’s well-known. So then you sell the rights to make it into a movie. Congrats, you’re rolling in dough… but now the studio takes over control of the story and the work begins to make it more fitting for the screen. Now some of these changes are obvious and necessary, such as cutting scenes to keep with a shortened run time or perhaps turning large blocks of dialogue into a more visual medium. This is to be expected with every story that makes its way to the big screen.
Books are taken through a lot of dialogue, that’s the medium, but movies are a visual medium and people want to see, they don’t want things described to them.
However… these changes are not always positive and some times they degenerate into a rather blatant form of meddling, adding more sex appeal to characters, perhaps adding in a comic relief character that wasn’t in the book or was a much less important character. Finally, product placement and merchandising comes into play. Once everything is done… the story can way too easily become unrecognizable from the original work.

3. Author interference.
In many cases, when we’re dealing with a more well-known author, extra steps have to be taken in order to get the rights to their books. The writers may make certain demands that the studios will have to work around. They also may often want to appease the author afterwards as there is no greater death sentence to a movie than a poor review from the person who wrote the original story.

This is one area that I’d caution about. If your film does get looked at to be turned into a play or a movie, be careful! Is the money you’re being offered worth having your story cannibalized? Most would say yes, but just make sure what you’re getting into, make sure that you can have your name pulled if you don’t like it, and make sure to have a lawyer read over the contract and negotiate things that you don’t want changed. Otherwise… you end up like these people…

Michael Ende:
This is one of my most beloved movies from my childhood. Imagine my surprise when I found out that the theme of the movie is actually ass-backwards from that of the book it was based on. It turns out that the book was condemning people who get lost in a fantasy world whereas the movie promoted the idea, saying that people no longer dreamed and thus were destroying themselves.
Ende absolutely loathed The NeverEnding Story to the point that he did everything he could until his dying day to destroy the movie. Apparently, the producers of the film hired other writers to do the above-mentioned edits to Ende’s script… completely neglecting to tell him anything about it. Ende was furious when he found out, but because the studio already owned the rights, he was largely ignored.
Ende was so angry that he had his name removed from the movie and proceeded to throw a multi-decade temper tantrum in the form of lawsuit after lawsuit. Ironically as the movie franchise became little more than a $1.99 at the gas station movie bin filler, starring Hollywood nuisance, Jack Black, by the time the 3rd movie came out, makes Ende’s anger seem… almost understandable.

P.L. Travers
I have never heard of anyone hating the movie that was based on their story more than her! She actually hated Walt Disney’s Mary Poppins more than I did!
By no means did she want her beloved story made into a movie and didn’t want Walt Disney’s… tackiness anywhere near it. Walt wouldn’t acquire the rights to MP for nearly 2 decades. It wasn’t until she fell on financial hardship that she finally surrendered the rights. Even so, she fought hard against the movie in almost every aspect. She hated the animated penguins, the senseless made up words, Julie Andrews and Dick Van Dyke, even the color red was objected to. Rumor has it she was also deeply offended by Dick Van Dyke’s accent!
After seeing the finished product, she was so distraught that she went to tear Disney a new one, but there was nothing she could do.
As she began to get on in years, Travers gave her consent for a stage version of Mary Poppins under the very strict condition that absolutely no one who worked on the Disney film could be involved in any way, shape of form. Furthermore, she wanted  only British writers to be hired to write it.
Rumor has it that she went so far as to attack Walt Disney in her will. Making her hatred of the film known one last time… eesh.

So clearly you can see what happens when studios go out of control. So before parting with the rights over your intellectual work in any way shape or form, please know what you may be getting yourself into!