One of the worst pieces of feedback you can get on your WIP is that the pacing is off. Sometimes (rarely) that's an easy fix. Unfortunately, this is not usually the case. There are several things that can hinder good pacing in a story. It could be empty dialogue, scenes that are too long and drag out well past their importance, and even lack of a good setting.
Fortunately, these things can be fixed, even if they do take some work.
Let's start with dialogue. It's important because reading page after page of internal thoughts and settings is booooring. But also boring is dialogue without any exposition. Here's an example:
"Hi, Sally. How are you?"
"Hi, Mark. I'm good. How are you?"
"Good. Good. Life is good. I'm so glad to hear that you're good."
"Well, it was nice to see you."
"And it was nice to see you."
Your eyes probably glassed over within a second of reading that dialogue. Why? Because 95% of it was unnecessary. It's repetitive, and well, seems to serve no purpose at all but to keep the characters talking. It's empty and painful to read. Don't do this. And if you do find yourself doing this, stop it. Delete it. Ask yourself what part of that dialogue is really necessary. Also, adding bits of imagery into the dialogue can really help to break it up. What are the characters doing? Is someone staring out the window? Taking a bite of food? Are they holding hands? Provide some sort of setting to the scene. (There is more on this below.)
Now, let's take a look at (too) long scenes. The best example I can think of here, is a fight scene, or a book that reads like one gigantic fight scene. You know the ones. There's a fight scene within the first 30 pages of the book and they just don't ever seem to end. The character finds him/herself constantly in battle and they just. never. seem. to. end. Even if the writing is well done, I can't take much of this. There needs to be some kind of build-up.
If the character is continuously repeating his or her actions throughout the story, it becomes boring and doesn't move the story along. It reads as more a placeholder and can turn a reader off very quickly. Don't get me wrong, fight scenes are important and they can certainly add action to a story that may be lacking in that area, but each scene in the book needs to hold some weight. There needs to be an altering moment in the scene, something that lets the reader know why this scene was important. If it doesn't appear to have a purpose to reader, it's just empty wording. Ask yourself: Why is this scene important? What does my MC learn from this scene? What does the reader learn? Could my story stand without this scene?
And now, let's set the scene. This (in my opinion) is one of the easiest fixes when it comes to pacing. Every story needs a setting. Every scene needs a setting. The setting can make or break a scene. If your reader can't read a scene and then tell you where the character(s) are, then you might have the problem of creating a bunch of white rooms. White rooms provide the reader with no context as to where the story is taking place and can really damage the reading experience. Each time you sit down to write a scene (or when you're revising and you're reading one) try to visualize where your character is and what he or she can see. Write that down. Write as much as you can. Look up images online to get ideas for how to describe the forest/room/closet/pool they are in. Maybe even sketch it out. You don't need ten paragraphs describing the scene, but a line or two here and there can really help set it up.
Obviously, these are just few examples of how to fix pacing problems in your story, but I hope they've helped you. Happy Writing!