time machine not backing up

Run the Console app (in /Applications/Utilities), and in the Filter field in the top right type in "backupd" (without the quotes). This will make it only show Time Machine messages.

Do you see anything in there to reveal what has gone wrong?

 

Ok, well that doesn't sound right. You should see at least some messages stating that a backup is starting. Are you certain that Time Machine is turned on and that it is starting (as in you see the clock icon in the menu bar spinning counter clockwise)? Otherwise, maybe you examined the console log just after it had been rotated to a new log file and Time Machine hadn't yet had a chance to do its thing.

Just to be sure, why don't you kick off a backup, wait for it to finish, and then go into Console and filter for backupd (while viewing "All Messages" or system.log). I know you've done this before, but there must be *something* in a log file to give us a clue!