When your flash is not working normally during photo sessions but it works when you hit the test button, you may have a problem with your hot shoe or sync cable.
The picture above shows the red arrow pointing to the nut to loosen the hotshoe box. Use it to make sure the box is all the way forward and secure it so that it can recieve the signal to trigget the flash,
The white arrow shows the hot shoebox in the pic is not fully secured and forward. After pushing it forward and tightening it the flash is now working for the customer.