I have a windows and a linux computer that are successfully syncing the files in their dropbox folder to the files I see when I log into dropbox online.
The problem occurred when I created an account (using the same email address) on a third (windows) computer - it does not sync at all. The files when logged in to dropbox online on this computer are the same as the other two, but the files in its dropbox folder are completely different and do not sync.
The weird thing is that the files in this computer's dropbox computer have the little dropbox icons on them (the ones that show whether they are synced, or just in the cloud, and so forth) - it's as if they are syncing to a completely different mystery repository from what I see when logged in online!
I have checked that I do have the desktop dropbox program installed (it's also clearly true, as the little dropbox icons show in the file browser), so this is not the problem.