The last backup it made was on July 23rd and I try to back it up manually now and it says 'Preparing backup..." but then eventually does nothing.
So you are saying if you leave whatever you have to back up like an external drive and it should and says will back up in an hour, I won't?Installed El Capitan a couple of days ago and now Time Machine is skipping scheduled backups even though there have been file changes. It works if I run it manually (from "Back Up Now" in the TM icon menu on the top line).