I'm not certain how you are attempting to push out updates, but there are two options that may work for you.
The first is to download the needed update from Apple's website. Once you expand the dmg the update should be in .pkg format. You can then use the "install package" option in ARD to push the package to your clients.
Alternatively, you can force the clients themselves to download and install the needed updates. You can do this by using the "send unix command" option and inputting the "softwareupdate" command with the required flags. You can read more about the softwareupdate command by either typing "man softwareupdate" in Terminal.app or you can visit this website:
http://www.manpagez.com/man/8/softwareupdate/