I have 5DII, an off-shoe camera shoe cord 2 from Canon (bought in about 1996) and a 3rd party flashgun.
3rd party flash gun works fine when mounted on top of camera.
When using the cord, the camera knows the flash is there as the flash symbol is shown on LCD through view finder when the flash is powered on and off when powered off.
Sometimes the flash triggers and sometimes it does not. It seems randon to me !! Any ideas?

