When I have flash-not-firing problems, it’s usually fixed when I clean off (using whatever’s handy, my shirt works most the time, but sometimes I may need alcohol to really scrub dirt off) the metal connection points on the bottom of the flash AND the metal areas on the hotshoe of the camera. I use my camera without a flash so often that the hotshoe and base of the flash will pretty dirty. Enough to interrupt the connection when I do connect them.
