Dear Romance Writer Girl
A friend (hi you-know-who) wrote to me recently and asked how is it that we come up with enough scenes to fill a whole book. She's been having trouble finishing her projects because they seem to stall out somewhere short of a complete novel.
I've suffered through this exact same problem, and it's one of the many reasons having written a book is much more fun than actually writing one.
The solution isn't easy, but there is a solution (and no, it's not giving up writing and becoming a used car salesgirl). Stalling out generally means there's something missing, and you have to go back to your story premise to figure out what it is. You need to make sure your characters have sufficient goals, motivations and conflicts to carry the story. Once you've got that settled, you can decide what scenes need to be added (or lengthened) to illustrate what was missing before.
Example: I wrote a book last year during a stressful time (when I wasn't thinking all that clearly), and for the life of me I could not get the book to come out at the length I needed it to be. I knew something was missing, but in my stressed-out haze couldn't figure out what it was. It took my brilliant editor reading the story to notice that, um, Jamie, the heroine doesnt have any goals.
Well, duh. It seems like such an obvious thing. The hero had goals, and the heroine had conflicts with him, but her lack of goals left her as sort of a bystander in the story. She didn't have any reasons to move forward. She was just there letting me tell her what to do. So I revised the story, and it came out much stronger--and longer--than before.