It seems Time Machine only checks the size of a file and not the date modified to back up. I have a Windows Virtual box and it does not back it up even though the date modified has changed since the last backup (because the virtual box file (.vdi) has not changed size).
Any ideas? Is there a setting to force Time Machine to check the date as well?
Was it ever backed up at all? As far as I know, a changed timestamp should include it.
However, your virtual machine image might also have been excluded using the extended attribute com.apple.metadata:com_apple_backup_excludeItem on the file. Use the xattr command in Terminal to see extended attributes.
See Does Apple's Time Machine app really copy everthing for more details.
You can use Apple's tmutil to control which files/directories are backed up by using this command to allow them to be backed up (if they're excluded):
sudo tmutil removeexclusion your_file_or_dir
One can also exclude files/dir using this command:
sudo tmutil addexclusion your_file_or_dir
