I have never used one of these, can this be the problem?
"Lockable: Press the button 5 times within 1.5 seconds to lock or unlock the battery. The battery LED will blink when switching modes. When the battery is in lock mode, the button is disabled and the battery will not power an atomizer or cartomizer."
Since I have never used one, I don't know if it will have any display in "locked" mode or not. My gut is that you have used it for 4 weeks already and know enough to check for this, I'm shooting in the dark here.
I agree with Sthur, check to see if the vendor will replace it under warranty if it isn't related to the lock. Drop a note when you figure this out, I'm interested to hear how it's resolved.
Good luck!