Occasionally the photos don't upload until I've opened up the dropbox camera upload folder on the pc. Also it can take anything from 20 seconds to well over a minute for a picture to appear in that folder.
Have you checked on the Dropbox website to see if it appears on there straight away? As it backs it up to the cloud first before syncing it to your PC.
I really don't know! Doesn't make sense but It's happened too many times for it to be coincidence but i can't 100% discount it. Phone doesn't seem to have upload issues as it's on wifi and i can upload via wifi by other means without issue.