Progress: The only type of failure I have been able to think of is a grounded LED in the string that has 6 lit up LEDs, so those "downstream" from that grounded one get no current through them, and the other string gets too little current to light up, since most of the current goes to the lower total voltage drop string with the ground.
Hip, hip, hurrah! Thanks to the troubleshooting tips from an experienced LED hobbyist on the Barr Report, I found the problem! First, I disconnected the ground connection to the string that didn't light up at all. Plugged it in, and nothing changed. So, I reconnected that ground, and disconnected the ground to the string that had the 6 still operating LEDs. Plugged it in, and the same 6 continued to be the only ones lit up. This had to mean there was a grounded LED in that string. So, I removed the mounting screws for the last lit up LED and placed a piece of paper behind it to keep if off the heatsink. Plugged it in, and the previously unlit string came on fully, and the string that had the 6 still lit ones was dark. Absolute proof that that LED was shorted to ground at the "-" end of it. I still can't see anything wrong on it, nothing to show that it is grounded, but there is no question that it was grounded. To double check this, I then reconnected both grounds, so both strings had the "-" end grounded, but left the faulty LED separated from the heat sink. Plugged it in, and both strings light up fully.
My tentative guess is that moisture between the star mount and the heatsink may have been involved, or the star mounting is not well done by the assembler. Tomorrow I will figure out what next. Any suggestions? Is there a relatively easy way to electrically isolate that one - with a mica shim, for example? Or would it then overheat?
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