Go back to the beginning.
Often a story stalls because you just haven’t given your protagonist enough to do. Making an adjustment at the beginning can vault you forward in the middle. For example, imagine you’ve decided to write a romance. Your protagonist is a young woman named Molly who lives in New York City. She goes on some dates, she falls in love, and she gets married. You could probably write some fun scenes here, but you might find yourself running out of steam by about page 140. It’s all a bit meandering. What if we charge things up a bit? What if we start the story by writing that Molly wants to get married? She’s the same Molly, but now she has a specific goal. Now she has to do something. We could also give her a timetable. Let’s say Molly wants to get married by the time she’s 30. And her birthday is next month. Immediately I’m feeling more anxious. What will happen if she doesn’t get married by her birthday? How is she going to find a man so quickly? What is she going to do? Do you see how one small quest invigorates the whole story? Now, instead of a string of anecdotes, we have a quest. What does your protagonist want? Maybe the Iron Throne? Maybe world peace? Maybe to fit into a size 6 dress?