Here’s a piece of brilliant advice on how to build your plot, from Trey Parker and Matt Stone, the creators of South Park and The Book of Mormon.
Don’t plan out your action linearly: she did this and then she did that and then she did something else and so on. She did this, but something happened, so she had to do that (completely unexpected). If you can fit “and then” when in your scene, rewrite it so you can replace it with “but”.
This works particularly well when you have to deal with those saggy middles, where not much happens.