Most likely, the front jacks are wired to turn off the rear output when something is plugged in. This feature was intended for instances where the user wanted to use headphones through the front and no output from the main speakers so as to not disturb others.
There are several ways around this, but it's all dependent on what specific equipment you have, what exactly you want to do, and how much you are willing to spend on extra stuff to accomplish it. Of course, your level of technical and electrical knowledge has some bearing, as well.