I have several files in my local Dropbox directory which Finder shows with the blue "in progress" indicator, some have been there for days.
How can I force Dropbox to Sync the unsynced files?
Check for the Dropbox icon in the Menu Bar. While the files are syncing, the icon is shown overlaid with a pair of arrows. Once the Sync is complete the overlaid arrows are no longer shown on the icon.
There is apparently a long standing bug in Dropbox client where even after files have Synced, Finder shows files as being synced with a blue sync arrow shown.
To make sure that files have Synced properly, relaunch the Dropbox client (click on the Menu Bar icon and select Quit Dropbox), and check on the Sync progress by looking at the Menu Bar icon. This is the most definitive test. Clicking on the Menu Bar icon also shows the current download and upload speeds along with the number of files to be synced and estimated time remaining.