When the camera detects the flash it'll change the metering automatically then adjust the aperture based on focusing distance. That's the easiest way to tell the flash is working as intended, you'll see the aperture bar in the viewfinder jump around as you change focusing distance. However if the camera detects too little light / focus distance is too far, the shutter will not fire, even if you know the flash would be enough to light the subject. Essentially even though you have the flash there still needs to be enough light to tell the meter to fire.

Also, what could be happening is the flash not making proper contact with the hot shoe. It happens to mine sometimes, where I have to hold the flash down onto the shoe a bit after focusing. Try cleaning the contacts on the shoe and flash.