Are you set to upload to dropbox only on wifi, and have you checked your wifi connection?
It could also help to disconnect your phone from dropbox in the dropbox settings, close the app, reopen it and reconnect. Sometimes the pipelines get clogged and this clears them out. For the same reason, you can try going to your Settings-Apps-Dropbox and clearing the cache (don't clear the data - that's unnecessarily drastic for now).
I wanted to take of wireless transfer only, but there's no option I can access at this point in the settings - it says it's uploading but I've been waiting at least 45 minutes for something to happen.
Dropbox also pauses uploading when the battery gets low - I'm not sure what the threshold is.
