One of mine has been a little uncooperative on the charger. Charger is good, the MCU on the battery isn't giving the MCU on the charger a battery-good condition to start the charge cycle. Watch for the battery's LED to flash, and the charger LED should change from green to red AFTER the battery's LED flashes a few times.
I've got it to behave once by leaving it off the charger for a couple hours, and the second time, I left it off for a couple hours, it didn't wake back up, so a put a cartomizer on it, took a couple draws, got it to flash a dead state, then put it on the charger and it started charging.
If it keeps happening, I'll diagnose further. My initial suspicion, since the MCU is obviously still good, is that the cell is getting drawn down below what a Li* cell should get pulled to, and will no longer trip the MCU in the charger... If that is the case, it's a bad thing since it will "soften-up" the cell over time. The "fix" is to not pull it down as far before
throwing it back on the charger.
[Sorry - hardware/firmware-engineer-chick speaking here.

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