I connected Dropbox to my nation via Settings > Apps > Dropbox. At first the only thing to sync were a bunch of old email themes and then it stopped syncing all together.
To troubleshoot I've disconnected and reconnected. Per NationBuilder support I double checked to make sure that NationBuilder is fully authorized in Dropbox.
We have received multiple reports of slowness related to Dropbox files syncing. We will begin looking into this issue but don't have a timeline for a specific fix.