A golden rule of writing: if your manuscript is getting stale, put your heroine in more trouble.
It's great to follow, but I always loosely translated that as:
Throw some bombs at her!
Kill off her family!
Have her blinded by toxic nerve gas!
That's all well and good, and exciting to be sure, but can sometimes be a little too much. What the rule really means is to push your heroine's desires further away from her, which could be a bomb or the sudden onset of blindness due to toxic gasses, or something much less dramatic. Basically, don't give her what she wants. Make her work for it and get the reader involved in her struggle.
Some of my favorite books are the ones where I'm practically shouting at the heroine to do something different. Of course, some of my least favorite ones are books where the heroine continually makes stupid decisions. There is a very fine line between being cautious and being a moron. So the challenge is to make things harder for your heroine but have her respond to new developments in a logical way.
In my current MS, my heroine is starting to like someone who is completely wrong for her. When she finally admits it to one of her friends, the boy overhears her confession. In the following chapter he confronts her about it and I had two choices: he could profess his undying love for her OR he could give her the "just friends" speech.
I wrote this chapter twice, using both scenarios. The thing us, you WANT the two of them to hook up, but the chapter I liked better was the one where he gave her the "just friends" speech. Had I let them skip off into blissful relationship happiness, what reason would you have to keep reading?
This is what I mean by making trouble for your characters. Simple trouble. Relatable trouble. Who hasn't liked someone only to find out they're in the "friend zone?" When my heroine finds out, she's stunned. Her ego takes a major blow. This is something we've all felt, and in addition to dashing my heroine's expectations, I make you, the reader, like her more because you can empathize with her.
If I had blinded her, you might feel sorry for her sudden loss of vision, but it wouldn't be something you would understand. (Unless it has happened to you.) So if your story is starting to drag in the middle, try giving your girl some normal troubles like:
Lied to by a friend!
Dumped by a boyfriend!
The target of the mean girl's bullying!
Parents getting divorced!
Dad in jail!
And if those don't work, by all means set off a gas bomb.