By default, TrueCrypt container files never change size and their last modification date & time never changes. This means the only way to detect that they have changed is to enable the "Use slower but more reliable method of file change detection" option for your SyncBack profile. However, that can significantly increase the backup time as it must compare the backup file with the original. An alternative option is to change a setting in TrueCrypt so that it changes the last modification date & time of the container file if its contents is modified. To do this select Settings > Preferences... from the main menu of TrueCrypt, and then untick the checkbox "Preserve modification timestamp of file containers".