It's difficult to diagnose this sort of problem without seeing the kit. Also unfortunately the manual for your housing is not available online.
If I understand correctly you have an Ike housing with your camera installed and the hot shot cable inside the housing pushed fully into the hot shoe in the direction of the arrow. You then have a Sea & Sea strobe connected to the strobe port on the housing by a Sea & Sea to Ike cable part number
4118.1
When you set the strobe to manual and the slave switch to off the strobe doesn't fire when you press the shutter. However if you set the slave switch to on then the strobe will sometimes fire.
If this is how you've got it setup the fact that it fires in slave mode proves the slave is working. Youu can test this outside the housing by setting the strobe to slave and pointing the camera flash at it. This means that the most likely problem is the cable. There is some information on testing Ike cables here
SYNC WIRING it may help but doesn't include information on the Sea & Sea bulkhead. It should allow you to test fire the strobe though.