It's not writer's block, per se...
What exactly is the middle of your book? "The middle follows the part where your character's motivations were established, their goals were set in place, and where your character decided to go after what he/she wanted or to fight some something he/she believed in...to reach a destination or prevent something from happening."
I've eliminated a few words for conciseness, but the meaning is still Cheryl's. In other words, "The middle is simple a series of events that gets your character from the beginning of the book to the end."
People who plot have it over those of us who don't. Most use plot points (an event that takes place and forces the character, willing or not, into new circumstances or direction. Things like:
The villain appears.
A letter arrives.
A love scene,
You're usually halfway when your character's goals change. Whatever your main character started out wanting should have changed direction by now, or he/she has come up with a new plan to get what they want. A complication makes it look like they will never achieve their goal. Don't make the mistake of not being mean enough to your character. Conflict is good, but remember, a delay in reaching a goal is NOT conflict.
Help yourself by making a list of 25 things that could happen and review when need be.
Make sure to keep the tension strong and heighten it when necessary.
Keep the outcome in doubt. Use a time limitation, but give the reader flashes of hope.
Change POV and leave your main character's fate hanging (a suspense technique), or add an action scene, but make sure you intersperse action with scenes of less tension for pacing's sake.
End every charter with a hook, to keep the reader turning pages.
Question the purpose of every scene. Is it really needed to move the story forward?
Make sure you haven't revealed too much about your characters.
Can the reader identify? Are you making the story believable?
Is the conflict escalating? Things should be worse than they were to start with.
Don't let your story become predictable.
Have you paid attention to pacing?
Is the sexual tension still high? If not, punch it up.
A good example comes from the movie, "Shrek." Characters are like onions...reveal them one layer at a time. If in reviewing your work, create a use later file, cut and paste into it to prevent telling too much too soon.
These are just a few suggestions from Cheryl's book. I urge you to check it out on Amazon. I don't plot, but I still found this an enormous help.