There is a command you can try.
follow that with
sudo rm /var/lib/apt/lists/* -rf
The first command removes/deletes all the sources scripts/files in the lists folder.
sudo apt-get update
The second command will replace those scripts. This works. I had to do it some months ago when I was unable to update because one of those files had a missing header.
The scripts in the lists folder contain information about all the packages installed. Update Manager compares the information in those scripts with the information in the repositories to work out which packages on the machine can be updated.
It seems that if there is something wrong with those scripts then Update Manager will not update.
Another reason for this problem could be at the other end, in the repositories themselves. When that happens it usually get put right in a couple of days.