What happened there? Do you think someone used the lantern as well as the Dot-It? If so, and if your son put it away knowing it was now dim, I'd have a word with him about it.

Or did the Lithium AA's under-perform? Or does the lantern draw a tiny amount of current even when off? I agree with the comments on rechargables (other than Li-Ion or Eneloops etc), but if Lithiums are failing we need to query Energizer.

I've been in the situation where I've prepared for power failure, finally got one, only to find my preparations didn't work. That's how I discovered tea-lights aren't as bright as proper candles. (This was back before LEDs.) It's galling, but educational. In theory, theory and practice are the same. In practice, they are different.