Provided you use the same Linux Mint 14 ISO image file to create the bootable USB pendrive and the bootable DVDs, then there is no functional difference between the Linux Mint14 live system on the USB pendrive and and on the DVDs.
So the difference must be somethings else, e.g. something which you are doing in a different way when booting from the USB pendrive on the one hand and when booting from the DVDs on the other hand.
One vague idea:
On the pendrive you might be starting the live system with a persistence file. In this persistence file the configuration details how to connect to the internet have been saved once and are being used automatically whenever you boot your machine(s) from the USB pendrive and try to access the internet.
On the DVDs there is no persistence file, and you do not execute all necessary steps in order to connect to the internet. (whatever the exact steps may be in your case)