Polk Audio Boat Tower Speakers not working?

I just recently bought a used Super Air Nautique 230 with just under 100 hours. It has four tower speakers and a 12" sub custom installed from the previous owner. Today i was listening to the sound system and I noticed that two of the tower speakers were not working. I check the connections at the amp and they were connected. I am getting sound out of the stock sound system as well as the sub, but only two of the four tower speakers are working. Any ideas??