Just took the camera out of the housing and shot it past the bare strobe sensor into the mirror. It syncs!
So does that likely just mean it's a cable issue (i.e. the cable isn't transmitting enough light)? If so, great, I'll look at trying a new cable.
A lot of times there's a small break near the connector end. If you have a removable connector, you can just cut off the bad end and replace the connector. You just need to be able to see the light shine through if you hold one end up to a strong light.
If it seems to be ok, try cleanning off the inside of the housing optical window or seeing if anything is blocking the light from shining through. We just had a customer have awful problems, until we cleaned off the inside of the little connector window with some alcohol. Problem solved!
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