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The camera hotshoe can become loose after much use with a flash. A loose hotshoe can cause the flash unit to intermittently not fire or fire on its own. The flash unit may also intermittently not communicate with the camera, and hence lose its ETTL (Canon), iTTL (Nikon), or TTL (conventional) flash metering.
Loose connections between the camera hotshoe and a hotshoe adaptor. Make sure the adaptor is tightly connected to the camera.
If the hot shoe has broken, this can be replaced.
The camera hotshoe can become tarnished by contaminants. A tarnished hotshoe can cause the flash unit to not communicate or even not fire at all. A tarnished hotshoe can be brushed/polished to regain its functionality.
The camera hotshoe can become corroded by liquids. A corroded hotshoe can cause the flash unit to not communicate and not fire at all. A corroded hotshoe should be replaced (not cleaned) to regain its functionality.