We're sorry you're having trouble syncing with Dropbox. Unfortunately when this happens, there is nothing we can do except off the following advice, based on our experience over the years:
Option 1: Switch to a different writing mode
The best thing to do if you're having trouble with Dropbox, is to stop using Dropbox, if that is an option for you.
You can switch to one of our other writing modes here:
leanpub.com/YOUR_BOOK/writing_mode
...making sure to replace YOUR_BOOK with your book's unique web address
Option 2: Try troubleshooting your Dropbox folder
One thing you can try is removing unneeded extra files from your Dropbox folder.
With any sync method, but especially with Dropbox, things can slow down if there are things like backups or large files present which are not needed. (We sync everything in your Dropbox folder.)
Another thing which can slow Dropbox down is if you are using Git in the Dropbox folder. One solution here is to use selective sync to avoid the .git directory.