I recently bought a home theater system, with a Yamaha receiver and Diamond 9.1 speakers. It worked okay, but I moved the receiver to another place, and no I don't hear anything in the front speakers (sub, 2 rear and center are working okay). I checked the connections, and all the parameters in the system setup and they seem okay (I haven't changed them when I moved the receiver).
Go through a process of elimination that your cable and speakers are not the problem. Switch the connections and see if they speakers work as back speakers, then switch the cables to the back to see if the cables work. Pare down all the possibilities until you get find what's causing the problem.