Well you probably know this but you have to separate the problem: the strobe, fiber optic cables, housing or the camera.
If you are with other divers or near a dive shop see if you can test your strobes on another system someone has. The same goes with the camera. With all the divers and cameras out there someone should be able to help you troubleshoot using their system.
Did you try swapping the batteries from the good working strobe to the one that wasn't? It may be that your charger for the batteries is not charging correctly and everything is else is okay.
Check the pop up flash and see if it is being interfered with inside the housing.
Also, if the flash on your camera is set at the lowest setting to save on the camera battery, try boosting it to maximum to see if it works while still in the housing. It's a simple quick check that will at least eliminate that potential problem.