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...The metering pre-flash would fire, but the actual flash either would not fire or was too underexposed to see/register. I tried a bunch of things to get it to work, but wasn't able to.
What shutter speed were you using? Third party accessories often won't work reliably at or near the max sync speed. 1/125 or 1/60 may work.
Also, to test if the exposure flash is actually firing you can set a long shutter speed and rear-curtain sync; that way there's a long delay between the preflashes and the exposure flash, making it easier to see.