If you've got a volt meter, measure the voltages on the testpads on the back. Measure the core voltage when it's first plugged in, and then when cgminer tries to start mining with it, and see if it changes. That'll let you know whether it's an ASIC issue or a power issue.
So I used my volt meter on the good stick, and got a reading. So I knew I was doing things right. I plugged in the "bad" stick, green light comes on. Check with volt meter shows 0. Start up cgminer. Still shows 0. Let cgminer do its thing, and try to hotplug. Still shows 0. Test the volt meter against my good stick, and I get readings.
So does this mean the asic is bad? Or maybe something blew? Like I said, all was good until I did a reboot of my PI the other day. Now only one stick is good.
Anything else I can try and test to see if I can diagnose it?