Replicator-G has a setting in preferences for the webserver where it gets its updates. The way it works is there if the firmware.xml file that lists all the updates and the settings the uploader (AVRdude) uses to flash the bot with.
Either, you aren't porperly connected to the internet so it can hit that website and update
or
You have the wrong URL
or
MakerBot didn't place the latest version on that website and that means Makerware is either using local copies or has an alternate update site.
The function is pretty simple.
And, why are you guys continuing to use stock firmware? Run Sailfish that is far more updated and has proven bugfixes and great support!!
Basically, there is NO EXCUSE to not run Sailfish.