MacBook Pro 17" (Snow Leopard) --> WD 2TB external drive
MacBook Pro 13" (Snow Leopard) --> Seagate 1TB external drive

I find that Time Machine sometimes doesn't back up new folders (and the files in them). This occurs both when I choose "Back Up Now" from the Time Machine icon in the Menu Bar and in TM's scheduled backups.

These are not excluded folders (nor are then in the TM do-not-back-up list); they're perfectly normal folders (at various locations) inside my home folder.

The only way to force them to be backed up is to restart the computer (unmounting & mounting the TM external disk does not help).

There seems to be a correlation with new folders (i.e., it's more likely to happen that an entire new folder is not backed up), but this may just be observer bias (because those are the folders that I go check to see if they've been backed up).

It's not computer dependent (it happens on two different computers). It's not external disk dependent (it happens on two different external disks). It's not time dependent (not restarting for several days does not fix the problem).

What does a restart change that these other events don't?

I'm considering deleting the /.fseventsd folder (without restarting the computer) to see if that helps.

I haven't tried logging out and logging in (without restarting the computer).