Dropbox won't sync files that are locked by another application. When Dropbox skips the file, the status message in your Dropbox menu (from the system tray or menu bar) will report that the file is in use. Don't worry, this is normal behavior.
Dropbox respects files that are open and in use, or locked, by another application. Applications lock files to prevent conflicts from occurring when the file is opened from one or more locations. For example, Microsoft Office (Word, PowerPoint, Excel, etc...) typically locks its files while they are open.
Simply save the file in question when you're done using it, exit the application, and Dropbox will start syncing it right away.