Sometimes people make changes to files in a Leanpub book's Dropbox folder, but they don't see any changes to their book when they make a new preview in Leanpub.
For a book you've been working on for a while, or that you've already published, here are the instructions for what to do next: http://help.leanpub.com/en/articles/2090395-i-have-a-weird-issue-trying-to-preview-and-i-m-using-dropbox-have-i-gotten-disconnected-somehow.
If you have just started a new book, and you have been doing some writing in files on your computer, and it is not syncing with Dropbox, the safest thing to do is probably to create a new book.
Here is what we suggest:
Back up any work you have done on your book on your computer
Create a new book here https://leanpub.com/create
Accept the Dropbox invitation to the new book's Dropbox folder
Make a change to one of the .txt files in the new book's manuscript folder, and then make a preview in Leanpub, to see if it works
If it does work, then copy the work from your back up, and paste it into the Dropbox folder for your new book
You can then delete the old book by going into the "Admin" section in the Book Tools menu for the old book, and then clicking "Delete Book"
You can rename your new book with the old book's title by going to the "Book Info" section in your new book's Book Tools menu, and then selecting "Book Info"
You can change the URL for the new book to the old book's URL (if the old book has been deleted, or you have changes its URL) by going to the Book Tools menu for the new book and selecting "Book Web Page" and then "Web Page URL"