There are so many variables as to why that is. I have seen headphone and microphone jacks on the front of computers that were never hooked up by the OEM where they were purchased. I've also seen them not provide the drivers to have them toggled on.
There are so many reasons why it may not be working. I would recommend contacting the system builder, whether it be a local computer shop or multi-national manufacturer, whichever be the case for you.
If you like, you can join the site and return to this thread to provide more information, like brand and model of computer, so that we can try to assist you further.