System Updates location/source question…

Hi All It seems as though instadmg only likes system updates when they come from apple. If I download say the 10.6.4 Combo Updater, check sum it and pop it in a workflow it errors but if it downloads from apple as per the example catalogs then there is no trouble. I've been putting it in the BaseUpdates folder and have tried the InstallerDiscs folder too but both fail. Anyone else have the same issues or am I doing something wrong? Do I need to flush the cache files perhaps? Regards Tristan