The camera hot shoe has a couple of leaf-springs on the rails and you slide in a camera flash (or anything that can slide into the hot shoe -- even if it's not a flash). Below one of the springs is a tiny micro-switch which is pressed down when anything is in the flash bracket. It should also pop back up when anything is removed from the bracket.
The camera will not raise the flash if it thinks something is attached to the hot-shoe.
However... if that button gets stuck, it'll think the hot-shoe has something attached even when there is nothing attached.
Thats ONE reason that a flash might not pop up.
Another is if the flash itself is stuck (e.g. something jamming the spring, etc.) But you'd know if it's jammed because the camera will display an error code (Err 5) ... after you hear a buzzing/clicking noise for a few seconds.
The flash is simply spring-loaded... and there's a catch-hook that retracts and lets the flash spring open. A solinoid pulls the hook away to release the flash (and that's the buzzing/clicking sound ... it'll try a few times to release and then display the error code if it doesn't detect the flash raised.)
If you are NOT getting that error code displayed, then it's probably the first issue (micro-switch/button is stuck under the leaf spring of the hot-shoe rail).
It would need to be cleaned. You can send it to Canon for service. If you try to do this yourself, do not lubricate it with oil (that'll just attract more dust and make things gunk up again). Typically just a tiny drop of alcohol can help dissolve whatever is jamming it (and the alcohold will evaporate).
Tim Campbell
5D III, 5D IV, 60Da
