My 5D has been slowly getting worse with using a flash.
The flash will fire, but not show any effect on the picture.
When shooting into a mirror the flash doesn't show in the picture at all, even though it fires.
Sometimes I could give the flash a push down on the camera and it will start working.
Now it doesn't help.
I have tried the flash unit on another camera and that is fine. Also have two flash units and both produce the same problem, yet work on the other camera.

Any ideas?

I can only assume something has broken on the cameras hot shoes...