backwpup folder on dropbox always empty

hello,
i’m having some problem, with backwpup.
on one hand it does seems that it’s working just fine. i’ve opened a dropbox account & its been Authenticated on backwpup. but when i’m running a backup job, nothing is being uploaded to the backup folder on dropbox. so actually i dont know if anything is working because i dont see any backup files.
I would appreciate any help with this.
thanks
Meitav

http://wordpress.org/extend/plugins/backwpup/