That sounds pretty frustrating! It must be because of "permissions." Do you know where your downloads are being downloaded (as in, the physical location of the folders) ?
Let's pretend they're in your /home/vivi folder, and that they're named "completed" and "incomplete".
Please post the output of
ls -l | grep -i incomplete
I bet that the owner and the group will be "root". So you will have to change them:
ls -l | grep -i completed
where "vivi" is your username. The one that is /home/vivi.
sudo chown vivi completed
also do a ls -l on the files inside the folders and see if they're owned by root. Also, you may have to give yourself read and write permissions.
For some homework, read up on the chmod, chown, and chgrp man pages by typing