There are a few possibilities, but Dropbox occasionally gets stuck syncing a specific file or folder for unclear reasons. One possible explanation for this is that there is a file conflict that Dropbox is unable to resolve.
If you click on the Dropbox icon and one file says "syncing" but the progress bar never moves, I recommend storing a copy of that file to your local system (outside of the Dropbox folder) and then deleting the problematic file from the Dropbox folder. This generally solves the issue for me.