This often happens when people have already done a lot of writing before they bring their Word file to Leanpub.
If the problem is at the beginning of your book, it may be because you have added a Table of Contents field in your book.
Leanpub generates a Table of Contents for you automatically, and our book generators are confused by Word's fields, including the Table of Contents field. You should save a new copy of your Word file, delete the Table of Contents, and create a new preview with that new Word file.
We describe all the formatting in Word our book generators are designed to handle in this blog post.