Semor Duncan twin humbies, 3 top string's work nothing out of the bottom 3? Power to both pick-ups? Changed the string's and nothing off the bottom string's? Been playing up a bit before string change on and off? Cheers regards Jim.
Yah, that's a stumper. Shouldn't even be possible with all things as usual.
Unless, as Christopher says, you have some sort of nonferrous strings on it.
when you say power, are these active pups? as in there is a battery in the guitar? that is a very odd issue unless you are using non magnetic wound strings some how